Add distance backend abstraction scaffold for issue #172 https://github.com/pysal/pointpats/issues/172

[SidheshwarSarangal]

This PR introduces a small distance-backend abstraction to decouple
distance computation from point pattern statistics.

What this does

• Adds a DistanceBackend interface
• Provides a default EuclideanDistanceBackend (no behavior change)
• Wires the backend into PointPattern without breaking initialization
• Updates a representative distance call in k()
• Adds a minimal unit test

What this does NOT do

• No network logic yet
• No API changes
• No refactoring of existing statistics

This is intended as architectural groundwork for network-constrained
point patterns discussed in #172.

Refs #172

[codecov]

Codecov Report

❌ Patch coverage is 58.06452% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (d3d09b2) to head (6e34488).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pointpats/distance_statistics.py	0.0%	8 Missing ⚠️
pointpats/pointpattern.py	76.9%	3 Missing ⚠️
pointpats/distance.py	80.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #175     +/-   ##
=======================================
- Coverage   72.6%   72.4%   -0.2%     
=======================================
  Files         12      13      +1     
  Lines       2124    2145     +21     
=======================================
+ Hits        1543    1554     +11     
- Misses       581     591     +10     

Files with missing lines	Coverage Δ
pointpats/distance.py	80.0% <80.0%> (ø)
pointpats/pointpattern.py	69.2% <76.9%> (+0.1%)	⬆️
pointpats/distance_statistics.py	15.0% <0.0%> (-0.5%)	⬇️

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