Port Bun CommonMark and extension specification corpus

Tracks the final Bun.markdown specification tranche under #66, following the edge-hardening work in #103 and GFM compatibility work in #104.

Scope:

• run the byte-identical js/bun/md/md-spec.test.ts fixture natively through Home
• port CommonMark block and inline semantics, GFM tables/tasklists/strikethrough/autolinks/tag filtering, and Bun wiki-link behavior
• preserve linear-time behavior for hostile nested links, wiki links, images, raw HTML, autolinks, emphasis, and reference labels
• keep all earlier Markdown fixtures green
• add a permanent exact-count Home regression
• pass the complete home_test gate

Validated locally before publication:

• headings: 17/17
• render callbacks: 37/37
• React: 68/68
• edge cases: 79/79
• GFM compatibility: 62/62
• specification matrix: 792/792
• failures/TODO/unsupported: 0 across every file
• full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: pass

Completed in c9c9930af. The byte-identical Bun fixtures now pass natively through Home: headings 17/17, render callbacks 37/37, React 68/68, edge cases 79/79, GFM compatibility 62/62, and the CommonMark/extension specification matrix 792/792—with 0 failed, 0 TODO, and 0 unsupported across all files. The implementation includes linear-time paths for deep nested links, wiki depth caps, nested images, long autolinks, references, and adversarial delimiter floods. A permanent 792-case Zig regression is pinned, fixture identity against ~/Code/bun was verified, and the complete ./pantry/.bin/zig build test -Dfilter=home_test --summary failures gate passes.