fix: clean up render processes without ps

[521Peter]

Summary

• replace the POSIX-wide ps scan with parent-scoped pgrep -P traversal
• probe process liveness with process.kill(pid, 0) and stop waiting at the cleanup deadline
• preserve a completed render result if best-effort cleanup itself fails
• add regression coverage for a host where ps is denied

Fixes #199

Verification

• node --import tsx --test packages/provider-hyperframes-local/test/capture-process.test.ts (3 tests passed)
• pnpm check (passed)
• git diff --check (passed)