Implement stopped-child wait4 semantics

[brandonpayton]

Summary

• add stopped-child process state and WUNTRACED/WNOHANG-aware wait4 polling
• propagate stop/continue behavior through Node and browser worker hosts
• write zeroed wait4 rusage structs and bump ABI_VERSION to 16
• document the supported wait4 stopped-child/rusage behavior

Validation

Specialist evidence recorded on kd-6z5:

• Node focused pcntl wait PHPTs: PASS 3/3
• Browser focused pcntl wait PHPTs: PASS 3/3
• Browser offset 9800 limit 100 refresh: 84 pass, 16 skip, 0 fail/time/error
• cargo test -p kandelo --target aarch64-apple-darwin --lib: PASS, 960 passed
• cargo test -p kandelo --target aarch64-apple-darwin poll_waitable_child: PASS, 5 passed
• PATH="$PWD/sdk/kandelo/bin:$PATH" npx vitest run host/test/process-wait-lifecycle.test.ts: PASS, 11 passed
• bash scripts/dev-shell.sh bash scripts/check-abi-version.sh: PASS
• git diff --check: PASS

Post-rebase checks run by mayor:

• git diff --check origin/main...HEAD: PASS
• bash scripts/dev-shell.sh bash scripts/check-abi-version.sh: PASS

Artifacts remain in the kd-6z5 worktree under test-runs/kd-6z5-phpt-after and are intentionally not committed.

[brandonpayton]

Closing as superseded by origin-headed replacement #759: #759

This PR head was in brandonpayton/kandelo-agentic-dev. The replacement uses the same head SHA (5ab7927) on an Automattic/kandelo origin branch, adds the [PHP] title prefix, and has an expanded description explaining why the pcntl/wait4 runtime fix is needed.