test: expand test coverage and improve code quality

Test Coverage Improvements:
- Added comprehensive error tests for incremental_insertion.rs:
  - Invalid vertex key, invalid facet cell, invalid neighbor wiring
  - Empty boundary facets, InsertionError::is_retryable() variants
- Expanded remove_vertex tests from single 3D test to dimension-parametric 2D-5D
- Added basic accessor tests for empty and simplex triangulations (2D-5D)
- Created dimension-parametric test macros for code reuse across dimensions

Test Consolidation:
- Consolidated test_local_facet_validation macro into streamlined test_facet_validation
- Reduced redundant tests from ~33 to 22 while improving coverage
- Each dimension now has 2 facet validation tests (detect + repair) instead of 5

Code Improvements:
- Added early success checks in repair loops using validate_facet_sharing()
- Optimization saves O(n·D) facet scanning when repair succeeds early
- Fixed clippy doc_markdown warnings (added backticks to function names)

Documentation:
- Streamlined WARP.md from 417 lines to 72 lines (83% reduction)
- Kept essential AI assistant rules, removed verbose explanations
- Improved focus on actionable guidance for code quality and testing

Author: acgetchell