Validate skill prerequisite ids — skills.json has dangling "requires" references

[StreamDemon]

P2 from the 2026-07-03 review (docs/reviews/2026-07-03-rules-layer-review.md, PR #19).

skills.json references three prerequisite ids that don't exist in the catalog: literacy, basic-mechanics, basic-electronics. Nothing validates requires referential integrity, so the dangling ids are silent today and would make any future prerequisite-checking feature silently wrong.

Fix (two parts):

1. Add referential validation — either a refine on skillCatalogSchema or a fail-fast check in buildSkillIndexes (matching the duplicate-id pattern).
2. Transcribe the three missing skills (all in RUE Skill Descriptions, printed pp.304–329) or correct the references.