fix: allow restarting a tracker after stop()

[davidberenstein1957]

What

stop() tore down both schedulers but never cleared _start_time, so a later start() returned early with "Already started tracking". The tracker looked started but was never sampled, and the next stop() wrote a row whose duration covered everything since the original start() — including the idle gap — making that row's duration and emissions_rate wrong.

Changes

• stop() records _paused_at after the final persist, and returns early (with the existing "Tracker already stopped !" warning) if the tracker is already stopped, instead of writing a duplicate row.
• start() rebuilds the schedulers when they were discarded, and on a restart shifts _start_time by the paused interval rather than re-stamping it. The energy accumulators are deliberately not reset on restart, so shifting keeps duration and the accumulators on the same clock: both cover active time only.

Note: a restarted run stays cumulative, and row 2's duration is now the sum of the active phases rather than wall-clock since the first start().

Verification

New test tests/test_emissions_tracker.py::TestCarbonTracker::test_tracker_can_be_restarted_after_stop — start/2s/stop, 2s pause, start/2s/stop — asserts the scheduler exists again after the second start() and that the second row's duration is ~4 s, not ~6 s. It fails on master and passes with this change. Full suite: 627 passed, 21 skipped.

Closes #1328

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.56%. Comparing base (065d0e6) to head (3faa49a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1337      +/-   ##
==========================================
+ Coverage   91.39%   91.56%   +0.17%     
==========================================
  Files          49       49              
  Lines        5056     5076      +20     
==========================================
+ Hits         4621     4648      +27     
+ Misses        435      428       -7     

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