fix: make tracker.stop() idempotent

[davidberenstein1957]

Calling stop() twice wrote a second complete emissions row (two CSV rows, two API POSTs, two handler.exit() calls) for a single run, because nothing recorded that the tracker had stopped — _start_time stays set and both schedulers being None was only used to emit an advisory warning.

What changed

• _initialize_runtime_state() now initializes _is_stopped, final_emissions and final_emissions_data.
• stop() returns early on _is_stopped, returning the cached final_emissions.
• The lock release moved after the guard, so a repeat stop() no longer retries os.remove on an already-removed lock file.
• The misplaced else: logger.warning("Tracker already stopped !") (bound to the _scheduler_monitor_power check) is replaced by the real terminal-state guard.

Why

__exit__ calls stop() unconditionally, so a with block plus an explicit stop() — the usual way to get the return value — double counts the whole run.

Verification

tests/test_emissions_tracker.py::TestCarbonTracker::test_offline_tracker_stop_is_idempotent asserts one CSV row and an identical return value after a double stop. It fails on master (2 rows) and passes with this change. Full file: 32 passed.

Closes #1307

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (065d0e6) to head (045655e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   91.39%   91.54%   +0.15%     
==========================================
  Files          49       49              
  Lines        5056     5063       +7     
==========================================
+ Hits         4621     4635      +14     
+ Misses        435      428       -7     

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