Port Bun shell lexer/parser testing APIs into the native corpus realm

Parent tracker: #66

Scope

• Expose reusable native shell lexer/parser serialization helpers without requiring a Bun-owned JSGlobalObject.
• Marshal tagged-template strings, primitive interpolations, raw fragments, arrays, and JS object references through JavaScriptCore C-API callbacks.
• Remove the Home-only TODO rewrites for js/bun/shell/lex.test.ts and parse.test.ts.
• Preserve the existing POSIX cp corpus coverage while installing the lexer/parser bridge.
• Add regression coverage and pass the full home_test gate.

Acceptance evidence

Landed in 911e577d9.

• Lexer corpus: 29 pass, 0 fail, 0 unsupported, 0 todo.
• Parser corpus: 18 pass, 0 fail, 0 unsupported, 1 upstream describe.todo.
• Existing cp corpus: 16 pass, 0 fail, 0 unsupported, 0 todo.
• Full home_test gate: 1054 total, 1053 pass, 1 intentional skip, 0 fail.
• Repository-wide Zig build passes.

Completed and pushed in 911e577d9. The native C-API bridge runs Bun shell lex/parse through Home’s real lexer/parser, removes both Home-only TODO rewrites, preserves 16/16 cp coverage, and passes the full Home gate.