fix(mobile): restore missing distance-transform diagonal

[tahazarif10]

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Summary

DistanceTransformPlanner.next() currently has seven neighbour offsets plus [0, 0], leaving the dy=-1, dx=+1 diagonal unavailable during path extraction.

The Euclidean distance transform itself is 8-connected, so this makes path extraction asymmetric with the cost map.

This replaces the self-offset with the missing diagonal [-1, 1] and adds a regression test on an interior 5x5 grid where that move is the unique shortest next step.

Related issue

Fixes #503

Checklist

• PR title follows Conventional Commits (type: description)
• Tests pass locally (pytest)
• Added/updated tests for this change, if applicable
• New/changed code is type-hinted with modern syntax (X | Y, list[X], not Union/Optional/List)
• Docstrings updated where applicable
• PR is as small/focused as practical
• No test files, data files, or notebooks specific to my own project

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (5b5a0fe) to head (835457e).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #644   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        143     143           
  Lines      14035   14035           
=====================================
  Misses     14035   14035           

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