rename track_id to tracklet_id for consistency

[TeunHuijben]

There was a severe inconsistency in the naming of the TRACKLET_KEY throughout the repo. This wasn't surfaced by the tests, because all conftest and pre-computed graphs had "track_id", and not any other name. This PR: 1) resolved the inconsistency, 2) updates all tests accordingly, 3) speeds up track_id calculation. Claude blurp:

The codebase had two strings for the same concept:

• DEFAULT_TRACKLET_KEY = "tracklet_id" — the public name exported from funtracks.annotators (what downstream callers like motile_tracker import).
• "track_id" — hardcoded as the default in Tracks.__init__, plus as string literals in validation, exports, and the annotator registry.

When a caller passed node_name_map={"tracklet_id": "...", ...} (the documented public spelling), the builder loaded the column under graph attribute "tracklet_id" but then called SolutionTracks(graph=...) without forwarding tracklet_attr=. Tracks.__init__ defaulted it to "track_id", didn't find that on the graph, and silently recomputed tracklet IDs from weakly_connected_components under a fresh "track_id" column. The user's original data was orphaned; tracks.features.tracklet_key pointed at the recomputed values — same number of unique tracklets, different assignments.

The fix collapses both failures: one canonical default everywhere (DEFAULT_TRACKLET_KEY), the builder forwards the key actually present in node_props to SolutionTracks(tracklet_attr=...), and a fallback in _setup_core_computed_features lets pre-2.0 saved data (still using the literal "track_id") load without recompute.

[codecov]

Codecov Report

❌ Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.48%. Comparing base (e7a7f13) to head (1d170fa).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/funtracks/import_export/_validation.py	75.00%	3 Missing ⚠️
src/funtracks/data_model/tracks.py	87.50%	1 Missing ⚠️
src/funtracks/import_export/csv/_import.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
+ Coverage   93.31%   93.48%   +0.17%     
==========================================
  Files          59       59              
  Lines        3217     3241      +24     
==========================================
+ Hits         3002     3030      +28     
+ Misses        215      211       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.