Fix utf8 crash

[piercefreeman]

While testing some more complex projects we've seen this error locally:

[2025-06-30T22:06:28Z ERROR firehot::layer] Error reading from child process stdout: stream did not contain valid UTF-8

monitor_stream was using std::io::Lines to read from child process stdout/stderr streams. The Lines iterator expects all input to be valid UTF-8, but child processes can output invalid UTF-8 bytes, causing the iterator to fail with a UTF-8 decoding error.