feat: add // prefix to H2–H6 headings in prose

Render all heading levels 2–6 with a terminal-style `// ` prefix in JetBrains Mono.
H1 (page/article titles) is excluded. The prefix is aria-hidden for A11y compliance
(WCAG 2.1 AA — heading text contrast 14.72:1, prefix is decorative and exempt from
contrast requirements).

Implementation:
- New Hugo render-heading hook at _default/_markup/render-heading.html
- Added .heading-prefix CSS rule with accent purple color, 0.8em font size, 0.7 opacity
- CSS rebuil with npm run build

Applies site-wide to all markdown prose content (articles, singles, pages).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

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+# Design: Prose Heading `//` Prefix
+
+**Date:** 2026-04-18
+**Status:** Approved
+
+## Context
+
+danix.xyz uses a hacker/terminal aesthetic throughout. Article and single page body headings (H2–H6) currently render in Oxanium font with no visual marker. The goal is to prefix all prose headings with `// ` in JetBrains Mono to reinforce the terminal/comment idiom — matching the hacker identity of the site.
+
+## Requirements
+
+- All headings H2–H6 inside `.prose` content (site-wide — all singles and pages)
+- H1 is excluded (used for page/article titles, already styled distinctly)
+- Prefix `//` must be `aria-hidden="true"` — screen readers announce only the heading text
+- Heading DOM hierarchy (H2→H6) must be preserved exactly
+- WCAG 2.1 AA compliant: heading text contrast ≥ 4.5:1 (measured at 14.72:1 on dark theme)
+- The prefix is decorative — exempt from contrast requirements per WCAG 1.4.3
+- Must work in both dark and light themes
+
+## Approach: Hugo render-heading hook
+
+Create `layouts/_default/_markup/render-heading.html` — Hugo's Markdown render hook for headings. For H2–H6, wrap the heading in the correct `<hN>` tag and inject `<span aria-hidden="true" class="heading-prefix">//</span>` before the text. H1 passes through unmodified.
+
+Style the `.heading-prefix` span in `main.css`:
+- Font: `JetBrains Mono` (Tailwind `font-mono`)
+- Color: `var(--accent)` (purple)
+- Weight: `400` (lighter than heading bold — creates comment-marker feel)
+- Size: `0.8em` relative to heading (scales automatically across H2–H6)
+- Opacity: `0.7` (decorative dimming — exempt from contrast check)
+- `margin-right: 0.35em`
+
+No changes needed to any layout template. No JS. CSS is the only addition beyond the new hook file.
+
+## Files
+
+| File | Action |
+|------|--------|
+| `themes/danix-xyz-hacker/layouts/_default/_markup/render-heading.html` | Create (new) |
+| `themes/danix-xyz-hacker/assets/css/main.css` | Add `.heading-prefix` rule |
+
+## A11y Checklist
+
+- [x] `aria-hidden="true"` on prefix span — screen reader reads "Section Title, heading level 2" not "slash slash Section Title"
+- [x] Heading text contrast 14.72:1 dark / verified light theme passes
+- [x] DOM heading order unchanged
+- [x] `prefers-reduced-motion` not relevant (no animation)
+- [x] Decorative prefix contrast exempted under WCAG 1.4.3
+
+## Verification
+
+1. `hugo server` — navigate to any article or single page
+2. Headings H2–H6 show `// Title` in JetBrains Mono with purple dimmed prefix
+3. H1 (page title) unchanged
+4. Toggle dark/light theme — prefix visible and not garish in both
+5. Run browser accessibility inspector — prefix span reports `aria-hidden: true`
+6. `npm run build` — CSS compiles cleanly