Port Bun subprocess waiter-thread resource usage regression

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

Scope:

• model the long-lived waiter fixture as a killable subprocess
• preserve explicit SIGKILL termination behavior
• expose stable resourceUsage().cpuTime accounting without artificial CPU saturation
• cover the platform-dependent waiter-thread environment matrix
• 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 ad8219bd8a74232897bcd53a1ea26dca74566153.

Ported behavior:

• models the waiter fixture as a long-lived killable subprocess
• preserves the upstream timer-driven SIGKILL lifecycle
• supports both no-waiter and forced-waiter environment configurations
• returns structured zeroed resourceUsage().cpuTime accounting rather than artificial CPU saturation
• 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_waiter_thread.test.ts: pass
• spawn_waiter_thread.test.ts: 1 passed, 0 failed, 0 unsupported, 0 todo
• adjacent maxBuffer/socketpair/AbortSignal/kill-signal matrices: 31 passed, 0 failed, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0