Fix sliceIntoLayers missing segments for near-parallel edges

[Copilot]

sliceIntoLayers fails to detect intersections when triangle edges are nearly parallel to the slice plane (< 1° angle), causing 0.6-5.8mm gaps in generated segments. This breaks path closure in 3D printing slicers like polyslice on geometries like Benchy.

Root Cause

Exact floating-point equality checks skip edges with distances like 1e-15:

continue if startDist is 0 and endDist is 0  # Misses near-zero values
if startDist is 0  # Never true for 1e-15

Changes

Angle-aware adaptive epsilon

• Calculate edge-plane angle via dot product with plane normal
• Scale epsilon by 100x when dotWithNormal < 0.02 (~1° threshold)
• Base epsilon scales with edge length: max(1e-10, length * 1e-9)

Replace exact comparisons

# Before
if startDist is 0

# After  
absStartDist = Math.abs(startDist)
if absStartDist < epsilon  # epsilon adapts to edge angle

Fix parameter validation

• Allow minZ == maxZ for single-plane slicing (was incorrectly rejected)

Test Coverage

Added 4 cases: near-parallel edges (2e-7 tolerance), tiny distances (1e-10), long edges (200+ units), duplicate prevention.

Example from new tests:

# Edge with vertex 2e-7 from plane now generates segment
vertices = [-1, -1, 0.2,  1, -1, 0.4999998,  1, 1, 0.7]  # V1 ≈ Z=0.5
layers = sliceIntoLayers(mesh, 0.1, 0.5, 0.5)
expect(layers[0].length).toBe(2)  # ✓ Passes with fix

All 507 tests pass (503 existing + 4 new).

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References

• polyslice PR #69 analysis identifying triangle 198780 with missing near-parallel edge
• See NEAR_PARALLEL_EDGE_FIX.md for detailed technical explanation

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Original prompt

We made some good progress upgrading the sliceIntoLayers method with #38 but we still have more work to do. To understand the problem we are trying to fix you can take a look at this PR jgphilpott/polyslice#69 but specifically this comment jgphilpott/polyslice#69 (comment)

You can also read the documents POLYTREE_PERFORMANCE_REPORT.md (https://github.com/jgphilpott/polyslice/pull/69/files#diff-17de4e77d834f0c1469ea64aad91bbf81a8bc2b7e035d037e6b96485b3c8dafa) and BENCHMARK_SUMMARY.md (https://github.com/jgphilpott/polyslice/pull/69/files#diff-7dd895eb123e2b8e6a021e643890496ff59ff174405d6e7e8b65ff3bff72a456).

Do you think you can investigate the issue and apply the recommended fixes for me?

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