feat: add diff syntax highlighting to code blocks

[denolfe]

Code blocks fenced as diff or patch now render with syntax highlighting. Added lines are green, removed lines are red, and changed lines (context-diff ! lines) are yellow.

```diff
-  return 'Hello, ' + name
+  return `Hello, ${name}!`
```

Key Changes

• Bundled diff grammar
  • Ships the tree-sitter-diff v0.2.0 wasm and its highlight query under assets/parsers/diff.
  • Registers diff, with patch as an alias.
• Diff line colors
  • New diff.plus, diff.minus, and diff.delta styles use the theme's green, red, and yellow.
• Vendored highlight query
  • The query drops the + - < > ! line markers from its punctuation.special capture. The markers then take the color of their line.
  • OpenTUI ignores the query's priority setting. With the upstream query, the marker renders grey on a green or red line.
  • parsers-config.json points at the local query. update-parsers keeps the edit when it regenerates assets.

References / Links

• tree-sitter-diff: grammar and upstream highlight query

[github-actions]

Startup benchmark (--render test/exhaustive.md, linux-x64)

build	mean	ratio	verdict
baseline (main@3350e91)	371.7ms ± 9.1ms	—
PR	371.4ms ± 4.1ms	1.00×	✅ ok

Thresholds: warn ≥ 1.1×, fail ≥ 1.25×. Baseline built from main.

First-frame (large.md, runs=10)

metric	baseline (main@3350e91)	PR	ratio	verdict
first-frame	284.8ms	283.4ms	1.00×	✅ ok

Thresholds: warn ≥ 1.1×, fail ≥ 1.25×. Baseline built from main.

Scroll (large.md)

metric	baseline (main@3350e91)	PR	ratio	verdict
full_mount	11958.0ms	11920.0ms	1.00×	✅ ok
step_p50	27.5ms	27.5ms	1.00×	✅ ok
step_search_p50	28.2ms	28.5ms	1.01×	✅ ok
stepMatch_p50	19.5ms	19.3ms	0.99×	✅ ok
commit	159.1ms	143.5ms	0.90×	✅ ok

Thresholds: warn ≥ 1.1×, fail ≥ 1.25×. Baseline built from main.