Port Bun subprocess kill signal matrix

Track the focused subprocess signal slice from #66.

Upstream byte-identical corpus file:

• js/bun/spawn/spawn-kill-signal.test.ts (85 lines, 16 tests)

Scope:

• activate the exact matrix instead of rewriting it to a native TODO
• model long-lived shell sleep subprocesses as pending until killed
• normalize default, named, numeric, null/empty, and NaN signal inputs
• expose OS signal constants needed by the upstream matrix
• preserve exitCode === null and canonical signalCode after signal termination
• reject unknown/non-coercible signals with ERR_INVALID_ARG_TYPE and enumerate valid signal names

Acceptance:

• exact matrix: 16 pass, 0 fail, 0 unsupported, 0 todo
• copied corpus remains byte-identical to ~/Code/bun
• full Home gate passes
• commit/push directly to main as Chris Breuer with no coauthors
• close this issue with evidence and link it from #66.

Completed and pushed directly to main in af7b89b (fix(runtime): port Bun subprocess signal matrix).

Implemented a reusable pending subprocess model for long-running shell sleeps, canonical signal normalization for default/named/numeric/null/empty/NaN inputs, OS constants.signals, stable signal-termination state (exitCode === null, canonical signalCode), and Node-compatible ERR_INVALID_ARG_TYPE errors that enumerate real signal names. Invalid kills leave the child pending so a later valid kill still settles the original exited promise.

Verification on final source:

• exact byte-identical js/bun/spawn/spawn-kill-signal.test.ts: 16 pass, 0 fail, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0
• durable Home regression reruns the exact matrix and asserts 16/0/0/0.