Port Bun subprocess IPC channel matrix

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

Scope:

• advanced and JSON serialization modes
• child-to-parent callback identity and message delivery
• parent send with child pong response
• IPC after a non-pipe extra stdio slot
• malformed advanced-frame length overflow safety
• unusable NODE_CHANNEL_FD teardown and error delivery
• durable exact-file regression

Acceptance:

• corpus remains byte-identical to ~/Code/bun
• all 8 active tests pass with 0 fail, 0 unsupported, 0 TODO
• cross-runtime and package-script IPC matrices remain green
• full home_test gate passes

Completed in daff0f5.

Evidence:

• Exact upstream spawn.ipc.test.ts remains byte-identical to ~/Code/bun.
• Exact matrix: 8 pass, 0 fail, 0 unsupported, 0 TODO.
• Advanced and JSON serialization modes both cover child-to-parent callback identity, parent send() and child pong response, and IPC after a non-pipe extra stdio slot.
• The malformed 0xFFFFFFFB advanced-frame length is rejected without crashing the receiver.
• Unusable NODE_CHANNEL_FD teardown emits ERR_IPC_CHANNEL_CLOSED, reports the IPC startup error, reaches the scheduled ok, and exits 0.
• Cross-runtime Bun/Node IPC pair and package-script IPC inheritance remain green.
• Full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0.
• Whole-file TODO rewriting was removed and a durable exact-file 8/0/0/0 regression was added.