Port Bun.markdown GFM compatibility semantics

Tracks a focused Bun parity slice under #66 and #81.

Scope:

• activate the unchanged js/bun/md/gfm-compat.test.ts fixture
• support GFM table interruption, width validation, escaped pipes, empty bodies, and alignment
• implement the disallowed raw-HTML tag filter without exposing escaped angle brackets
• match URL/email autolinks, punctuation balancing, single/double strikethrough, fenced blocks, and heading delimiter precedence
• add an exact permanent regression and run the complete home_test gate

Completed on main in 93ef0a562.

Evidence:

• fixture is byte-identical to /Users/chris/Code/bun/test/js/bun/md/gfm-compat.test.ts
• exact unchanged GFM fixture: 62 passed, 0 failed, 0 unsupported, 0 todo
• earlier Markdown fixtures remain green at 17/17, 37/37, 68/68, and 79/79
• permanent Home regression asserts the GFM 62/0/0/0 result
• final ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed with zero failures
• git diff --check: passed

Implemented coherent GFM table parsing, tag filtering, URL/email autolinks, single/double strikethrough, fenced HTML rendering, and heading delimiter precedence.