Port Bun spawnSync timeout and event-loop isolation semantics

Scope

Activate and pass the copied Bun fixtures:

• js/bun/spawn/spawnsync-no-microtask-drain.test.ts
• js/bun/spawn/spawnsync-isolated-event-loop.test.ts

Required behavior

• Apply Bun.spawnSync({ timeout }) as an absolute monotonic deadline while draining child pipes.
• Terminate and reap timed-out children without losing already captured output.
• Report Bun-compatible exitCode, string signalCode, and exitedDueToTimeout.
• Keep timers and microtasks from draining inside synchronous child execution.
• Execute compound nested -e programs through the real Home runtime instead of a broad synthetic process.exit() shortcut.
• Add exact-corpus repository regressions and pass the full Home test gate.

Tracks the next bounded tranche of #66 and #81.

Implemented and published on main in 1e5491c590e2163b1e3adf6711be4226259c2aac (feat(test): port spawnSync isolation semantics).

What landed:

• native spawnSync pipe draining now uses one absolute monotonic timeout deadline
• timed-out children receive SIGTERM, are drained and reaped, and preserve captured output
• results expose Bun-compatible string signalCode plus exitedDueToTimeout
• the legacy eval shortcut now only intercepts scripts consisting solely of process.exit(n), allowing compound child programs to execute in Home
• both copied Bun isolation fixtures are active with permanent exact-corpus regressions

Verification:

• spawnsync-no-microtask-drain.test.ts: 3 pass, 0 fail/todo/unsupported
• spawnsync-isolated-event-loop.test.ts: 4 pass, 0 fail/todo/unsupported
• existing spawnSync.test.ts: 5 pass, 0 fail/unsupported, 1 pre-existing todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed with zero failures.