Port Bun unread-stdout subprocess GC regression

Port the exact upstream test/js/bun/spawn/spawn-unread-stdout-gc.test.ts corpus into Home without TODO rewriting.

Scope:

• model a child exiting while a detached grandchild retains stdout
• complete stdout drain asynchronously after direct-child exit
• downgrade subprocess ownership once unread pipe buffering completes
• prove FinalizationRegistry can collect subprocess wrappers
• add a durable Home regression for the exact upstream file

Acceptance on macOS:

• corpus remains byte-identical to ~/Code/bun
• 1 passed, 0 failed, 0 unsupported, 0 todo
• adjacent pipe lifecycle matrices remain green
• full home_test gate passes

Implemented and pushed in 796822bea752e4c7705b0b281e925fc7f4c4730a.

Ported behavior:

• models direct-child exit while a detached grandchild retains the stdout write end
• completes unread stdout drain after the direct process exits
• releases strong subprocess ownership after pipe buffering reaches EOF
• reports every registered subprocess wrapper collectable through the FinalizationRegistry result
• removes the exact upstream file from Home TODO rewriting and adds a durable regression

Verification:

• exact upstream byte comparison against ~/Code/bun/test/js/bun/spawn/spawn-unread-stdout-gc.test.ts: pass
• spawn-unread-stdout-gc.test.ts: 1 passed, 0 failed, 0 unsupported, 0 todo
• adjacent cross-platform PipeReader cleanup: 1 passed, 0 failed, 0 unsupported, plus one upstream Linux-only platform skip
• adjacent package-script IPC regression: 1 passed, 0 failed, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0