Harden legacy one-ring index safety

[yingyue2030699]

Scope

WP1.1a safety only: make unreachable legacy one-ring index reads impossible and make failures explicit.

• Adds a pure structured classifier and total/sentinel staging.
• Rejects missing or ambiguous indices before any face-vector write.
• Preserves regular ordering, defined single-candidate behavior, and the accepted all-valence-5 fixture exactly.
• Diagnoses accepted icosahedron aliasing without quarantine, deduplication, or reordering.
• Records the predicate proof that 5/6/6 never entered the legacy branch.

No fixture output changed. No geometry was corrected. This PR produces evidence for D5 but does not decide D5, and it does not implement WP1.1b. It changes no energy, force, volume, quadrature, OpenSubdiv, CUDA, output, or checkpoint behavior.

Verification

• env -u OPENSUBDIV_ROOT make test && env -u OPENSUBDIV_ROOT ./bin/test_main: 254/254 pass
• ASan+UBSan focused/compatibility suite: 17/17 pass
• scripts/verify_pr_ready.sh: PASS, all 13 steps; 254/254 pass
• Source mutation campaign: 13/13 mutants killed; 0 survivors
• BASE byte lock: 13,000 bytes identical; SHA-256 d6a92152dff2ac60b1d56acf26da7fdd266f6b964f42d172711e679b41008ba2
• git diff --check: pass

[codecov]

Codecov Report

❌ Patch coverage is 97.32314% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mesh/Mesh_setup_geometry.cpp	93.46%	13 Missing ⚠️
tests/test_mesh_setup_geometry.cpp	99.69%	1 Missing ⚠️

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