Port Bun subprocess maxBuffer and timeout matrix

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

Scope:

• add spawn-pipe pipeTo() interoperability required by TextDecoderStream
• terminate async and sync subprocesses at finite maxBuffer with the requested signal
• expose exitedDueToMaxBuffer for sync results
• enforce finite async and sync timeouts with exitedDueToTimeout for sync results
• preserve unlimited output and runtime behavior for Infinity
• add a durable Home regression for the exact upstream file

Acceptance:

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

Implemented and pushed in d720e3fc5eca6d2d6e65b4a7d461ba28ddb4f39e.

Ported behavior:

• adds spawn-pipe pipeTo() interoperability and async iteration for transform readable streams
• terminates async and sync subprocesses at finite maxBuffer with the requested signal
• exposes exitedDueToMaxBuffer and exitedDueToTimeout on sync results
• models finite timeout timing and preserves unlimited output/runtime behavior for Infinity
• corrects shared sync timeout validation so Bun-compatible Infinity is accepted while invalid finite values remain rejected
• 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/spawn-maxbuf.test.ts: pass
• spawn-maxbuf.test.ts: 8 passed, 0 failed, 0 unsupported, 0 todo
• adjacent socketpair/AbortSignal/kill-signal matrices: 23 passed, 0 failed, 0 unsupported, 0 todo
• existing spawnSync.test.ts: 5 passed, 0 failed, 0 unsupported, 1 pre-existing todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0