Retain weighted predecessors in many-to-many path queries

[antonsoo]

The many-to-many branch of path2d computes weighted predecessors, then discards them and calls unweighted hillclimb2d. With cardinal cost 1 and diagonal cost 3, this can replace two cheap cardinal steps with an expensive diagonal.

Return pf.path_from(best_ij) after choosing the nearest start. Keep endpoint selection and unreachable handling. Equal-cost routes may choose different tied predecessors.

The regression covers four diagonal settings. An independent heap solver on the bundled sample dungeon confirms all 59 previously incorrect many-to-many queries, with all 1,680 single-endpoint path controls unchanged. A separate automatic-heuristic correction handles another 60 single-endpoint failures; composing both fixes matches all 2,240 queries.

Full headless tests pass (325 passed, 95 skipped), using the pinned resource submodule and SDL dummy audio driver. Mypy and applicable project formatting/hooks pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.09%. Comparing base (e974570) to head (5762316).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #197   +/-   ##
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  Coverage   86.09%   86.09%           
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  Files          29       29           
  Lines        7597     7597           
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  Hits         6541     6541           
  Misses       1056     1056           

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