refactor: restructure repo into src/ site/ dist/ vendor/ packs/

Separate framework source from website:
- src/layers/ + src/main.css: CSS framework source (was assets/css/)
- site/: Hugo website (content/, layouts/, hugo.toml)
- dist/: built output (asw.css, asw.min.css)
- vendor/open-props/: vendored dependency with version tracking
- Hugo module mounts: dist/ → static, site runs from site/

Build: hugo --source site/ passes (105 pages).
npm run build produces dist/asw.css.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

site/content/notes/diff.md

@@ -0,0 +1,58 @@
+---
+title: "Diff"
+description: "Render structured diffs with line-level semantic markup — added, removed, context, hunk."
+section: notes
+prev-url: "status/"
+prev-title: "Status"
+next-url: "session/"
+next-title: "Session Log"
+type: notes
+date: 2026-04-09
+tags: ["notes", "diff", "reference"]
+ai-disclosure: "generated"
+ai-model: "claude-sonnet-4-5"
+ai-provider: "Anthropic"
+
+---
+
+## Overview
+
+Use `data-diff` on a `<pre>` block to render a structured diff. Child elements carry line-level semantics.
+
+```html
+<pre data-diff>
+  <span data-line="hunk">@@ -12,7 +12,9 @@</span>
+  <span data-line="context"> function greet(name) {</span>
+  <span data-line="removed">-  return "Hello " + name;</span>
+  <span data-line="added">+  const greeting = `Hello, ${name}!`;</span>
+  <span data-line="added">+  return greeting;</span>
+  <span data-line="context"> }</span>
+</pre>
+```
+
+## Line Types
+
+| Value | Meaning |
+|-------|---------|
+| `hunk` | Hunk header (`@@ ... @@`) — muted, italic |
+| `context` | Unchanged surrounding line — default text |
+| `removed` | Deleted line — red background, `-` prefix |
+| `added` | New line — green background, `+` prefix |
+
+## Inline Diffs
+
+For single-value changes in prose, use `data-diff` directly on `<del>` and `<ins>`:
+
+```html
+<p>
+  Latency changed from
+  <del data-diff="removed">240ms</del>
+  to
+  <ins data-diff="added">18ms</ins>
+  after the index rebuild.
+</p>
+```
+
+## Notes
+
+The `data-diff` block expects monospace rendering. ASW applies `font-family: var(--asw-font-mono)` automatically — no additional class needed.