Port Bun.markdown adversarial edge-case semantics

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

Scope:

• activate the unchanged js/bun/md/md-edge-cases.test.ts fixture in the native Home corpus runner
• match HTML, callback, React, and ANSI behavior for adversarial links, images, references, headings, tables, entities, and terminal controls
• keep bracket, backtick, HTML-opener, autolink, emphasis, reference, and table-width floods bounded and stack-safe
• preserve Bun Buffer construction semantics efficiently enough for spawned stress fixtures
• add an exact permanent regression and run the complete home_test gate

Completed on main in 7d8d25ec3.

Evidence:

• upstream fixture is byte-identical to /Users/chris/Code/bun/test/js/bun/md/md-edge-cases.test.ts
• exact unchanged fixture: 79 passed, 0 failed, 0 unsupported, 0 todo
• permanent Home regression asserts the same 79/0/0/0 result
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed with zero failures
• git diff --check: passed

The port covers callback/React/HTML/ANSI edge semantics, stack-safe nested images and links, bounded reference/wiki/table parsing, terminal-control sanitization, linear adversarial input paths, and bulk Buffer/ASCII decoding needed by the real spawned stress programs.