Enhance Teensy serial handling and refactor code for clarity

[CeliaBenquet]

This pull request adds a new developer-facing documentation page that explains the experiment session lifecycle, and makes a series of style and clarity improvements to the DeepLabCut processor code. The documentation addition provides a clear, step-by-step overview of how the two main processes interact during a session, while the code changes focus on improved logging, exception handling, and minor refactoring for readability.

Race-condition fixes

TeensyLatency.close_serial():

• added timeout=0.5 to the serial.Serial(...) constructor so readline() can never block indefinitely, and close_serial() now joins the reader thread before closing the port. Also done in Fix thread crash in TeensyLatency + prevent data-loss for dlc_inference_w_pd_sync #329.
• same fix applied to Teensy.py (the main rig Teensy, not just the photodiode one) and to the two DLC-position socket clients, dlc_socket.py/dlc_deque_socket.py (conn.close() racing a blocked recv()), since they're the same failure shape.
  ActiveSensingTask.stop() now closes its dlcClient (and joins its thread) so the position-reading socket/thread don't leak across repeated task start/stop cycles in one GUI process.

Documentation Improvements:

• Added a new page, docs/software_package/experiment_lifecycle.md, detailing the session lifecycle, process responsibilities, data flow, and failure handling for developers. This is now included in the table of contents (_toc.yml).

vr4mice GUI save/close safety (teensy_experiment.py)

• _dump_data(): save failures now show an error dialog instead of an uncaught traceback.
• Fixed a latent bug: check_close()'s "did you save?" prompt was a silent no-op once saved_ok was already True — the Close button did nothing in that case.
• The window's [X] button and Ctrl+C in the terminal now route through the same save-check (previously both bypassed it entirely and skipped Teensy cleanup on interrupt).
• New "unsaved data" warning on clicking "Ready" (which replaces self.task, discarding the old task's in-memory data) — not a hard requirement, since not saving can be intentional (test/aborted runs).
• Fixed another latent bug found while adding that: self.saved_ok was never reset to False after a new task was created, so after the first save in a GUI session, every subsequent unsaved-data warning silently stopped firing.