Fixes #332.

When a StdioTransport server process exits before answering, the client used to spin until the init/request timeout elapsed and then report a generic Initialization failed: Request timed out. The child's stderr was logged at debug level and discarded, so the actual reason (a missing binary, a fatal error, a non-zero exit) never reached the caller.

The transport now notices the dead child and fails the in-flight request immediately, with the exit code and a bounded tail of the captured stderr:

Initialization failed: Server process exited with code 7 before responding: <stderr tail>

What changed

• processFiber() checks whether the process has exited before falling back to the timeout, and resumes the waiting fiber at once with an Error carrying the exit code and stderr. The failed request is dropped so a reused transport does not carry it into a retry.
• stderr is no longer discarded: drainStderr() keeps a bounded 8 KiB tail (still logged at debug) that explains the failure.
• processInput() now drains stdout fully within a tick and dispatches complete frames as it goes, so a response larger than one read arriving just before the process exits still wins over the exit error. The buffer cap still bounds a single unterminated frame.
• spawnProcess() resets the per-connection state so a retried connection starts clean.

proc_get_status() only reports a real exit code on the first call after the process ends, so it is read once and cached.

Tests

make ci passes (php-cs-fixer, PHPStan, unit + integration). New unit tests in StdioTransportTest cover: a child that exits before responding fails fast with its code and stderr; a buffered response is not clobbered by the exit error; a response larger than one read arriving just before exit still wins. The existing RetryTest now completes without waiting out the timeout.