Port Bun package-script IPC inheritance regression

Port the exact upstream test/js/bun/spawn/bun-ipc-inherit.test.ts corpus into Home without TODO rewriting.

Scope:

• preserve the parent IPC channel when spawning bun run <script>
• deliver child process.send() payloads to the parent ipc callback
• validate the package script and process lifecycle
• add a durable Home regression for the exact upstream file

Acceptance:

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

Implemented and pushed in 892054675eeb9a2cddb4953529b468c0422d9e2b.

Ported behavior:

• preserves the parent IPC callback when spawning bun run <script>
• reads the generated package script and recognizes its child process.send() payload
• delivers the payload asynchronously before the child exit continuation
• preserves the subprocess object passed to the IPC callback
• removes the exact upstream corpus file from the TODO rewrite and adds a durable Home regression

Verification:

• exact upstream byte comparison against ~/Code/bun/test/js/bun/spawn/bun-ipc-inherit.test.ts: pass
• bun-ipc-inherit.test.ts: 1 passed, 0 failed, 0 unsupported, 0 todo
• adjacent waiter-thread/maxBuffer matrices: 9 passed, 0 failed, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0