Port Bun and Node cross-runtime IPC serialization

Port the exact upstream cross-runtime IPC JSON serialization pair into Home without whole-file TODO rewriting:

• test/js/bun/spawn/spawn.ipc.bun-node.test.ts
• test/js/bun/spawn/spawn.ipc.node-bun.test.ts

Scope:

• Bun parent with Node child
• Node parent with Bun child
• ordered parent/child startup, shutdown, and bidirectional message transcript
• empty stderr and successful exit lifecycle
• durable exact-file regressions

Acceptance:

• both corpora remain byte-identical to ~/Code/bun
• 2 active tests pass with 0 fail, 0 unsupported, 0 TODO
• adjacent package IPC inheritance remains green
• full home_test gate passes

Completed in 881494f.

Evidence:

• Both exact upstream corpora remain byte-identical to ~/Code/bun.
• spawn.ipc.bun-node.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO.
• spawn.ipc.node-bun.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO.
• Ordered parent/child start, end, child receive, and parent receive transcript is preserved with empty stderr and exit code 0 in both directions.
• Adjacent bun-ipc-inherit.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO.
• An initial full-gate run hit the existing load-sensitive maxBuffer timing threshold at 306 ms versus 200 ms; isolated spawn-maxbuf.test.ts immediately passed 8/8.
• Clean-state full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures retry: exit 0.
• Whole-file TODO rewriting was removed for both files and a durable two-file regression was added.