Extract single-particle likelihoods explicitly in Linker

[sylvesterkaczmarek]

Particles.draw_orbit() returns a one-orbit list, so Particles.lnlike() returns a one-element array. Assigning that array directly to lnL[j] raises ValueError: setting an array element with a sequence with NumPy 2.5.3, preventing the documented Linker + Particles workflow from sampling track assignments.

Extract the single likelihood explicitly with np.asarray(...).item(). Existing scalar-returning likelihood providers remain supported. Probability normalization and the small-likelihood fallback are unchanged.

Regression coverage uses real Particles objects with a deterministic likelihood fixture, covering the first track, multiple model choices, and the existing small-likelihood fallback. No changes to orbit selection or propagation.

Validation on Python 3.12 and NumPy 2.5.3:

• New regression tests on unmodified main: 4 failed with the reported exception. After the fix: 4 passed.
• Focused linker tests: 10 passed.
• Full suite: python -m pytest -q -n 2 --tb=short returned 310 passed, 2 skipped, and 35 subtests passed.
• New tests pass Flake8; no additional Flake8 diagnostics in linker.py relative to the base revision.
• Syntax checks and git diff --check passed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.32%. Comparing base (6af6000) to head (cc59145).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   97.32%   97.32%           
=======================================
  Files          17       17           
  Lines        5895     5895           
=======================================
  Hits         5737     5737           
  Misses        158      158           

Flag	Coverage Δ
unittests	97.32% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
ssapy/linker.py	100.00% <100.00%> (ø)

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[sylvesterkaczmarek]

Thanks for merging this one too. I really appreciate the quick reviews and I’m glad to keep contributing to SSAPy.