helix-fusion-lint-was-structural-only-now-semantic

Prior to this session, fusion-invariants-lint.py only validated section headers and taxonomy blacklist (structural shape) — it could not detect phantom skill references, dead MEMORY.md pointers, or incorrect vault tags. Three semantic checks were added: phantom-skill-ref (references to skills that don’t exist on disk), dead-docs-pointer (MEMORY.md topic-file links that resolve to missing files), and vault-tag hygiene (tags not matching the skill’s own slug). All 71 skills passed 0/0 post-extension.