Add boundary smoothing to polygon generation

[Kanahiro]

Summary

This PR adds Chaikin corner-cutting smoothing to polygon boundaries after simplification, reducing pixel jaggedness while maintaining computational efficiency and area accuracy.

Key Changes

• Boundary smoothing implementation: Added smooth_features() method that applies densification followed by iterative Chaikin smoothing to polygon geometries

  • Densifies boundaries to 3-cell segments before smoothing to cap corner rounding radius
  • Applies 2 iterations of smoothing with 0.25 offset for balanced visual quality and performance
  • Works directly on geometries rather than through processing algorithms for low per-click cost

• Updated test assertions: Adjusted area tolerance in multiple test files to account for slight area changes from smoothing

  • Changed from exact area assertions to pytest.approx() with rel=0.05 or rel=0.02 tolerance
  • Affected tests across test_polygon_maker.py, test_processing_algorithm.py, and test_preview_session.py

• Renamed and enhanced test class:

  • Renamed TestSimplification to TestSimplificationAndSmoothing
  • Renamed test method to test_staircase_boundary_is_thinned_and_smoothed
  • Added max_turn_degrees() helper function to verify that stair-step corners (90-degree angles) are eliminated
  • Updated assertions to verify smoothing effectiveness: max turn angle < 60° and vertex count ≤ 60

• Updated test documentation: Clarified comments explaining that both thinning and smoothing work together to produce the final boundary

Implementation Details

The smoothing is integrated into the build_polygons() workflow after the simplification/cleaning step. Three new module-level constants control the smoothing behavior:

• DENSIFY_INTERVAL_CELLS = 3.0: Controls maximum segment length before smoothing
• SMOOTH_ITERATIONS = 2: Number of Chaikin smoothing passes
• SMOOTH_OFFSET = 0.25: Chaikin offset as fraction of segment length

This approach balances visual quality (smooth corners) with computational efficiency and maintains area accuracy within acceptable tolerances.

https://claude.ai/code/session_014zaxaNkWQVvhjCkmHx9Tz8