Port Bun stdin pipe fd and FileSink ownership matrix

Port the exact upstream test/js/bun/spawn/spawn-stdin-pipe-fd-leak.test.ts corpus into Home without whole-file TODO rewriting.

Scope:

• close untouched stdin pipe ownership when the child exits
• retain a usable proc.stdin FileSink getter after exit
• balance FileSink wrapper ownership when stdin is accessed
• preserve the explicit upstream Zig/Rust divergence TODO
• add a durable Home regression for the exact upstream file

Acceptance on macOS:

• corpus remains byte-identical to ~/Code/bun
• 2 active tests pass, 0 fail, 0 unsupported
• 1 explicit upstream divergence TODO remains
• adjacent pipe lifecycle matrices remain green
• full home_test gate passes

Implemented and pushed in 7d322d4dd4329fe48681e537f053d34efac671f6.

Ported behavior:

• removes Home whole-file TODO rewriting so the exact upstream ownership assertions execute
• verifies untouched stdin: "pipe" descriptors close at child exit without requiring GC
• verifies proc.stdin remains a usable FileSink when first accessed after child exit
• locks in the shared stdin sink and reference-balanced fileSinkInternals.liveCount() behavior with a durable regression
• preserves the explicit upstream Zig/Rust divergence TODO unchanged

Verification:

• exact upstream byte comparison against ~/Code/bun/test/js/bun/spawn/spawn-stdin-pipe-fd-leak.test.ts: pass
• active tests: 2 passed, 0 failed, 0 unsupported
• explicit upstream divergence: 1 todo
• adjacent unread-stdout GC regression: 1 passed, 0 failed, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0