DR-078 — substrate_doc is kept, optional, and must resolve from the docs-site root

substrate_doc stays on initiative records as an OPTIONAL pointer holding depth the ledger’s budgets can’t; mirrors remain generated summaries, never the depth store. Every non-empty value must resolve from the docs-site repo root (preferred home for initiative-specific docs: src/content/docs/initiatives/{slug}/); pointers into git-ignored dirs or outside the repo are forbidden — CLEAR or relocate instead. Live reader: start-session §2c. Nothing enforces this — the register entry is the rule’s only home.

(Swept unrecorded at ec9fcfde; re-accepted by owner ruling S499 — the S499 tombstone audit found it still binding with no other home.)