fix: set PTY raw mode before process start

[lox]

Summary

• apply PTY raw mode before starting the child process, so early output cannot be transformed by the default line discipline
• keep PTY sizing and controlling-terminal setup in the PTY helper
• add regression coverage for output written before raw mode used to be applied

Problem

We were seeing a startup race in the pty-raw path.

The child process was being started before the PTY master was put into raw mode, so output written immediately on startup could still be processed with the default PTY line discipline. When that happened, \n was rewritten to \r\n, which made TestProcessOutputPTY_PTYRawExperiment fail intermittently.

Testing

• go test ./process
• go test ./process -run TestProcessOutputPTY_PTYRawExperimentWritesBeforeRawMode -count=1
• go test ./process -failfast -count=100 -run TestProcessOutputPTY_PTYRawExperiment

[DrJosh9000]

LGTM with one style comment split over two suggestions