Return 0.0 for degenerate input in simplex_volume_in_embedding

[basnijholt]

Summary

Fixes the last known behavioral divergence from the reference, the one called out in adaptive#493: simplex_volume_in_embedding raised ValueError on collinear/coplanar/coincident input where the reference returns 0.0.

This matters now that adaptive 1.5.0 auto-selects this backend: curvature losses feed degenerate vertex sets whenever the sampled function is locally flat, so LearnerND runs with curvature_loss_function() crashed with the Rust backend where the pure-Python backend returned a zero loss (the 7 known curvature test failures, plus test_learnernd_with_neighbor_aware_loss_runs in this repo).

What changed

The reference's tolerance contract is: a Cayley-Menger squared volume in (-1e-15, 0] is a degenerate input with volume 0.0; only below that band is it an error. This implementation raised for anything <= 0. Now:

• The general Cayley-Menger path returns 0.0 inside the band (new documented EMBEDDED_VOLUME_SQ_EPS constant in tolerances.rs) and raises only below it.
• 3 vertices in a >2D embedding (the curvature-loss shape) use the Cayley-Menger form expanded in squared distances instead of root-based Heron, so its rounding near zero behaves like the reference's numeric determinant and the same band applies. In 2D the reference's Heron path — which raises on any negative product — is mirrored unchanged.
• 2 coincident vertices in a >2D embedding return 0.0 like the reference's Cayley-Menger path (2D keeps raising, as the reference does for any 2-vertex input there).

Testing

• Full adaptive 1.5.0 test suite passes with the Rust backend active: 283 passed, 0 failures (previously 7 curvature failures). The 9 curvature tests pass individually too.
• This repo's suite: 133 passed with adaptive 1.5.0 installed, including the previously-failing test_learnernd_with_neighbor_aware_loss_runs.
• New Rust unit tests (collinear in 2D/3D, coplanar tetrahedron, coincident endpoints in 2D/3D) and Python parity tests, including 50 randomized near-collinear triangles asserting value-or-exception agreement with the reference.
• cargo clippy -D warnings, cargo fmt, ruff 0.11.0 clean.

Follow-ups this unblocks

1. Patch release (0.3.1).
2. adaptive-side PR: bump _MIN_RUST_VERSION to (0, 3, 1), add the Rust-backend CI job deferred in adaptive#493, and adopt the 0.3.0 batched APIs (simplices_containing, default_loss).