Port Bun.markdown render callback semantics

Track the deterministic port of Bun’s copied js/bun/md/md-render-callback.test.ts corpus.

Scope:

• callback-based inline and block rendering
• heading, link, image, code, list, table, and task-list metadata
• nested list callback ordering and indices
• entity decoding and parser options
• typed-array input lifetime while callbacks execute
• permanent exact-corpus regression and full home_test verification

Parent trackers: #66 and #81.

Implemented and published in f51d3e8cf.

Exact evidence:

• copied js/bun/md/md-render-callback.test.ts: 37 passed, 0 failed, 0 TODO, 0 unsupported
• inline callbacks: text, strong, emphasis, strikethrough, code spans, links, images, and autolinks
• block callbacks: headings, paragraphs, fenced code, rules, blockquotes, nested ordered/unordered/task lists, and tables
• Bun-compatible callback metadata and bottom-up nested-list ordering
• entity decoding and heading-ID parser options
• typed-array input transfer locking during callbacks, released after rendering
• permanent exact-corpus regression added
• full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures gate passed

This deterministic fixture is fully active and the blanket TODO is removed.