fix(Runner): add timeout for cache warmup and use posix_kill instead of exit

[s2x]

Summary

Fixes #140 - Runner hangs indefinitely when cache is empty.

Problem

When the cache was empty, Runner::run() would fork a child process to warm up the cache. However:

1. Using exit() in the child caused deadlock with extensions like grpc that register shutdown handlers
2. There was no timeout mechanism - if something went wrong during warmup, the parent would wait forever

Solution

1. Add 30 second timeout for cache warmup with clear error message
2. Use posix_kill(getmypid(), SIGKILL) instead of exit() to avoid deadlock
3. Handle SIGKILL properly - recognize it as expected exit when child kills itself
4. Throw descriptive RuntimeException on timeout

Testing

• All 293 tests pass
• phpstan no errors
• php-cs-fixer no issues
• rector no issues