Port Bun.markdown React element semantics

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

Scope:

• React and React 18 element symbols and Fragment structure
• inline/block Markdown element ASTs
• nested lists, blockquotes, tables, code blocks, links, and images
• component overrides with preserved props
• renderToString integration and HTML escaping
• permanent exact-corpus regression and full home_test verification

Parent trackers: #66 and #81.

Implemented and published in 0b5c7bd42.

Exact evidence:

• copied js/bun/md/md-react.test.ts: 68 passed, 0 failed, 0 TODO, 0 unsupported
• Bun default transitional and React 18 element symbols, Fragment shape, keys/refs/props
• inline and block element ASTs including nested emphasis, lists, blockquotes, tables, code, links, images, breaks, and entities
• string/function component overrides with prop preservation
• React-compatible createElement and renderToString integration with escaping and void elements
• permanent exact-corpus regression added
• full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures gate passed

The blanket React renderer TODO is removed and this focused issue can close.