Add opt-in file logging for freeze/debug diagnosis

open   by a73x

Waystty currently has no persistent runtime log file, which makes freezes hard to investigate after the fact. Runtime diagnostics mostly go to stderr/stdout, and if the app was launched from a compositor or app launcher there may be nothing useful left to inspect.

Proposal:
- Add opt-in file logging rather than always-on logging.
- If `WAYSTTY_LOG` is set, append structured log lines to the given path.
- Keep default behavior unchanged when the env var is unset.
- Log enough state to diagnose hangs and event-loop stalls without being excessively noisy.

Suggested scope:
- Initialize a process-wide logger early in startup.
- Record timestamps, level, subsystem, and message.
- Cover startup, Wayland connection/configure events, PTY read/write activity, resize/scale changes, render loop transitions, and fatal errors.
- Ensure logging failures do not crash the terminal.

Why opt-in:
- Avoids disk churn in normal use.
- Keeps the default terminal experience clean.
- Makes it easy to repro with `WAYSTTY_LOG=/tmp/waystty.log waystty` when debugging a freeze.

Open questions:
- Whether to also support a default XDG state path later (for example `~/.local/state/waystty/waystty.log`).
- Whether to buffer, line-buffer, or fsync on each write.