/* ═══════════════════════════════════════════════════════════
   El problema XY — hoja de estilos
   HTML/CSS/JS vanilla, sin dependencias externas.
   ═══════════════════════════════════════════════════════════ */

:root {
    --bg: #fbfaf8;
    --surface: #ffffff;
    --surface-alt: #f4f2ee;
    --ink: #1c1b19;
    --ink-soft: #56534d;
    --ink-faint: #7c7871;
    --border: #e2ded6;
    --accent-x: #2f5bd7;
    --accent-x-soft: #e8edfc;
    --accent-y: #b4530a;
    --accent-y-soft: #fbeee1;
    --focus: #2f5bd7;

    --measure: 34rem;
    --radius: 10px;

    --font-body: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    --font-mono: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, "Liberation Mono", monospace;
}

@media (prefers-color-scheme: dark) {
    :root {
        --bg: #15161a;
        --surface: #1d1f25;
        --surface-alt: #23262e;
        --ink: #e9e7e3;
        --ink-soft: #b3afa8;
        --ink-faint: #8b877f;
        --border: #32353e;
        --accent-x: #8fa9ff;
        --accent-x-soft: #232a41;
        --accent-y: #e9a463;
        --accent-y-soft: #33261a;
        --focus: #8fa9ff;
    }
}

*,
*::before,
*::after {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    scroll-padding-top: 5rem;
    -webkit-text-size-adjust: 100%;
}

@media (prefers-reduced-motion: reduce) {
    html {
        scroll-behavior: auto;
    }
}

body {
    margin: 0;
    background: var(--bg);
    color: var(--ink);
    font-family: var(--font-body);
    font-size: 1.0625rem;
    line-height: 1.65;
    -webkit-font-smoothing: antialiased;
}

.container {
    width: min(100% - 2.5rem, 62rem);
    margin-inline: auto;
}

a {
    color: var(--accent-x);
    text-decoration-thickness: 1px;
    text-underline-offset: 0.15em;
}

a:hover {
    text-decoration-thickness: 2px;
}

:focus-visible {
    outline: 2px solid var(--focus);
    outline-offset: 3px;
    border-radius: 3px;
}

.skip-link {
    position: absolute;
    left: -9999px;
    top: 0;
    background: var(--surface);
    color: var(--ink);
    padding: 0.75rem 1rem;
    z-index: 100;
    border: 1px solid var(--border);
    border-radius: 0 0 var(--radius) 0;
}

.skip-link:focus {
    left: 0;
}

/* ─────────────── Barra superior ─────────────── */

.topbar {
    position: sticky;
    top: 0;
    z-index: 50;
    background: color-mix(in srgb, var(--bg) 92%, transparent);
    backdrop-filter: blur(8px);
    border-bottom: 1px solid var(--border);
}

.topbar-inner {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    min-height: 3.5rem;
}

.brand {
    display: inline-flex;
    align-items: baseline;
    gap: 0.6rem;
    color: inherit;
    text-decoration: none;
    font-weight: 600;
}

.brand-mark {
    font-family: var(--font-mono);
    font-size: 1.05rem;
    letter-spacing: -0.03em;
    color: var(--accent-x);
}

.brand-slash {
    color: var(--ink-faint);
}

.brand-name {
    font-size: 0.95rem;
    letter-spacing: -0.01em;
}

@media (max-width: 26rem) {
    .brand-name {
        display: none;
    }
}

.lang-link {
    font-size: 0.85rem;
    color: var(--ink-soft);
    text-decoration: none;
    border: 1px solid var(--border);
    border-radius: 999px;
    padding: 0.25rem 0.75rem;
    white-space: nowrap;
}

.lang-link:hover {
    color: var(--ink);
    border-color: var(--ink-faint);
}

/* ─────────────── Portada ─────────────── */

.hero {
    padding: clamp(3rem, 8vw, 5.5rem) 0 clamp(2rem, 5vw, 3.5rem);
    border-bottom: 1px solid var(--border);
}

.kicker {
    margin: 0 0 1rem;
    font-size: 0.8rem;
    letter-spacing: 0.09em;
    text-transform: uppercase;
    color: var(--ink-faint);
}

.hero h1 {
    margin: 0 0 1rem;
    font-size: clamp(2.5rem, 8vw, 4.25rem);
    line-height: 1.05;
    letter-spacing: -0.035em;
    font-weight: 700;
}

.lead {
    margin: 0;
    max-width: var(--measure);
    font-size: clamp(1.125rem, 2.5vw, 1.375rem);
    line-height: 1.5;
    color: var(--ink-soft);
}

.lead strong {
    color: var(--ink);
    font-weight: 600;
}

/* ─────────────── Diagrama X / Y ─────────────── */

.xy-diagram {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    align-items: stretch;
    gap: 1rem;
    margin: 2.5rem 0 2rem;
}

@media (max-width: 40rem) {
    .xy-diagram {
        grid-template-columns: 1fr;
        gap: 0.75rem;
    }

    .xy-arrow {
        transform: rotate(90deg);
        justify-self: center;
    }
}

.xy-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.25rem 1.35rem;
}

.xy-card-y {
    border-left: 4px solid var(--accent-y);
}

.xy-card-x {
    border-left: 4px solid var(--accent-x);
}

.xy-label {
    margin: 0;
    font-size: 0.75rem;
    letter-spacing: 0.08em;
    text-transform: uppercase;
    color: var(--ink-faint);
}

.xy-letter {
    margin: 0.15rem 0 0.35rem;
    font-family: var(--font-mono);
    font-size: 2.25rem;
    line-height: 1;
    font-weight: 700;
}

.xy-card-y .xy-letter {
    color: var(--accent-y);
}

.xy-card-x .xy-letter {
    color: var(--accent-x);
}

.xy-text {
    margin: 0;
    font-size: 0.95rem;
    line-height: 1.5;
    color: var(--ink-soft);
}

.xy-arrow {
    color: var(--ink-faint);
    display: flex;
    align-self: center;
}

.see-also {
    margin: 0;
    font-size: 0.95rem;
    color: var(--ink-soft);
}

/* ─────────────── Rejilla de contenido ─────────────── */

.layout {
    display: grid;
    gap: 2.5rem;
    padding: clamp(2rem, 5vw, 3.5rem) 0 4rem;
}

@media (min-width: 60rem) {
    .layout {
        grid-template-columns: 13rem 1fr;
        gap: 4rem;
    }
}

/* ─────────────── Índice ─────────────── */

.toc {
    font-size: 0.9rem;
}

@media (min-width: 60rem) {
    .toc {
        position: sticky;
        top: 5rem;
        align-self: start;
        max-height: calc(100vh - 7rem);
        overflow-y: auto;
    }
}

.toc-title {
    margin: 0 0 0.6rem;
    font-size: 0.75rem;
    letter-spacing: 0.08em;
    text-transform: uppercase;
    color: var(--ink-faint);
}

.toc ul {
    list-style: none;
    margin: 0;
    padding: 0;
    border-left: 1px solid var(--border);
}

.toc ul ul {
    border: 0;
    padding-left: 0.85rem;
}

.toc a {
    display: block;
    padding: 0.3rem 0 0.3rem 0.85rem;
    margin-left: -1px;
    border-left: 2px solid transparent;
    color: var(--ink-soft);
    text-decoration: none;
}

.toc ul ul a {
    font-size: 0.85rem;
    color: var(--ink-faint);
}

.toc a:hover {
    color: var(--ink);
}

.toc a[aria-current="true"] {
    color: var(--accent-x);
    border-left-color: var(--accent-x);
    font-weight: 600;
}

/* ─────────────── Texto ─────────────── */

.prose {
    min-width: 0;
}

.prose > section + section {
    margin-top: 3.5rem;
}

.prose section section {
    margin-top: 2.25rem;
}

.prose h2 {
    margin: 0 0 1rem;
    font-size: clamp(1.5rem, 3.5vw, 1.95rem);
    line-height: 1.2;
    letter-spacing: -0.02em;
    scroll-margin-top: 5rem;
}

.prose h3 {
    margin: 0 0 0.75rem;
    font-size: 1.15rem;
    letter-spacing: -0.01em;
    scroll-margin-top: 5rem;
}

.prose p {
    max-width: var(--measure);
}

.prose b {
    font-family: var(--font-mono);
    font-weight: 700;
    font-size: 0.95em;
}

/* Secuencia de pasos del problema */

.secuencia {
    max-width: var(--measure);
    margin: 1.5rem 0;
    padding: 0;
    list-style: none;
    counter-reset: paso;
}

.secuencia li {
    position: relative;
    counter-increment: paso;
    padding: 0 0 1.1rem 2.5rem;
    color: var(--ink-soft);
}

.secuencia li::before {
    content: counter(paso);
    position: absolute;
    left: 0;
    top: 0.1rem;
    width: 1.6rem;
    height: 1.6rem;
    display: grid;
    place-items: center;
    border-radius: 50%;
    background: var(--surface-alt);
    border: 1px solid var(--border);
    color: var(--ink-faint);
    font-size: 0.8rem;
    font-variant-numeric: tabular-nums;
}

.secuencia li:not(:last-child)::after {
    content: "";
    position: absolute;
    left: 0.8rem;
    top: 1.85rem;
    bottom: 0.15rem;
    width: 1px;
    background: var(--border);
}

/* Consejos */

.consejos {
    max-width: var(--measure);
    margin: 1.5rem 0;
    padding: 0;
    list-style: none;
}

.consejos li {
    position: relative;
    padding: 0 0 0.9rem 1.75rem;
    color: var(--ink-soft);
}

.consejos li::before {
    content: "";
    position: absolute;
    left: 0.15rem;
    top: 0.72em;
    width: 0.5rem;
    height: 0.5rem;
    border-radius: 2px;
    background: var(--accent-x);
}

.consejos strong {
    color: var(--ink);
}

.remate {
    margin: 1.5rem 0 0;
    padding: 1rem 1.25rem;
    background: var(--accent-x-soft);
    border-radius: var(--radius);
    color: var(--ink);
    font-size: 1rem;
}

/* Transcripción IRC */

.transcripcion {
    margin: 1.5rem 0 0;
    padding: 1.15rem 1.25rem;
    background: var(--surface-alt);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow-x: auto;
    font-family: var(--font-mono);
    font-size: 0.875rem;
    line-height: 1.7;
    tab-size: 4;
}

.transcripcion code {
    font: inherit;
    color: var(--ink-soft);
    white-space: pre;
}

/* Diálogo citado */

.dialogo {
    margin: 1.5rem 0 0;
    padding: 0.35rem 0 0.35rem 1.25rem;
    border-left: 3px solid var(--accent-y);
}

.dialogo p {
    margin: 0 0 0.7rem;
    color: var(--ink-soft);
    font-size: 0.975rem;
}

.dialogo p:last-child {
    margin-bottom: 0;
}

.dialogo b {
    font-family: var(--font-body);
    font-weight: 600;
    font-size: 1em;
    color: var(--ink);
}

/* Fuentes */

.fuentes {
    font-size: 1rem;
    color: var(--ink-faint);
}

.fuentes a {
    margin: 0 0.3rem;
}

.nota-traduccion {
    font-size: 0.9rem;
    color: var(--ink-faint);
}

/* ─────────────── Pie ─────────────── */

.footer {
    border-top: 1px solid var(--border);
    padding: 2rem 0 3rem;
    font-size: 0.9rem;
    color: var(--ink-soft);
}

.footer p {
    margin: 0 0 0.4rem;
}

.footer-meta {
    color: var(--ink-faint);
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
}

/* ─────────────── Página 404 ─────────────── */

.error-page {
    display: grid;
    place-items: center;
    text-align: center;
    padding: clamp(4rem, 15vh, 8rem) 0;
}

.error-code {
    margin: 0 0 0.5rem;
    font-family: var(--font-mono);
    font-size: clamp(3rem, 12vw, 5rem);
    line-height: 1;
    letter-spacing: -0.04em;
    color: var(--accent-y);
}

.error-page h1 {
    margin: 0 0 1rem;
    font-size: clamp(1.5rem, 4vw, 2rem);
    letter-spacing: -0.02em;
}

.error-page p {
    margin: 0 auto 1.75rem;
    max-width: 30rem;
    color: var(--ink-soft);
}

.boton {
    display: inline-block;
    padding: 0.65rem 1.4rem;
    border-radius: 999px;
    background: var(--accent-x);
    color: #fff;
    text-decoration: none;
    font-weight: 600;
    font-size: 0.95rem;
}

@media (prefers-color-scheme: dark) {
    .boton {
        color: #15161a;
    }
}

.boton:hover {
    filter: brightness(1.08);
}