refactor: Surface duplicate boundary facets as errors instead of silent filtering

Replace silent deduplication with explicit error handling to catch algorithmic
bugs in cavity boundary detection early.

**Breaking API Changes**
- deduplicate_boundary_facet_info: Returns Result<SmallBuffer, InsertionError>
  instead of Vec (errors on duplicates, uses SmallBuffer for performance)
- filter_boundary_facets_by_valid_facet_sharing: Returns Result<SmallBuffer>
  instead of Vec (propagates errors from facet map building)
- Add InsertionError::DuplicateBoundaryFacets variant with duplicate_count and
  total_count fields

**Rationale**
The cavity boundary should form a topological sphere with no duplicate facets.
Duplicates indicate:
- Incorrect neighbor traversal logic
- Non-manifold mesh connectivity
- Data structure corruption

By returning errors instead of silently filtering, we surface these bugs
immediately rather than allowing corrupted triangulations.

**Performance Optimization**
- Use SmallBuffer (stack-allocated, falls back to heap) for typical D+1 facets
- Faster than Vec for small collections (common case in D dimensions)
- Maintains O(n) duplicate detection with FastHashSet

**Implementation Details**
- Propagate DuplicateBoundaryFacets errors through insert_vertex_cavity_based
  and create_cells_from_facet_handles
- Update tests to expect errors on duplicates instead of silent filtering
- Rename prop_deduplication_is_idempotent to prop_deduplication_detects_duplicates
- Fix test_create_cells_from_facet_handles_duplicate_handles to assert error

Author: acgetchell

proptest-regressions/core/traits/insertion_algorithm.txt

@@ -5,3 +5,4 @@
 # It is recommended to check this file in to source control so that
 # everyone who runs the test benefits from these saved cases.
 cc 87a07747781317977c0434e7fa3b29a415e54b0b22c6f3510657229a4daf0bbc # shrinks to vertices_processed = 1, cavity_failures = 0, fallback_uses = 2
+cc 49467c2716ef7d552c20f2c532e469f9093e68440f5e5f352c8bb588402a7b4f # shrinks to count = 1