MR-8 — Invariants are numbered and stable

Project invariants in CLAUDE.md are I-1, I-2, … . Once assigned, the number is stable forever. If an invariant is repealed, the number is not reused — it gets marked repealed with a pointer to the repealing decision doc. New invariants get the next available number.

Code comments and decision docs cite invariants by number: (I-3). Changing the number breaks the back-references.