Schema: hydration as a regenerable index over the identity trail — never a snapshot-as-self (ADR-0032 §2.2.3)

Problem

Consumers (the render-model, the FM cockpit, any "who is this resident now?" query) need a fast way to read an identity's current state. The naive answer — snapshot the current state as "the self" — is the Fork-8 trap: the snapshot goes stale the instant the resident acts or its model changes, and every migration forks a new stranger. The identity substrate needs a hydration mechanism that is a regenerable view over the lossless trail, not a frozen self. Depends on the node-types (#14693).

Intended shape (ADR-0032 §2.2.3, normative)

• Hydration = a regenerable index computed over the ordered EmbodiedEpisode trail — the head era + a projection of the anchor + social layer. It is a view, not a source of truth.
• Losing the index loses nothing — it is rebuildable from the trail at any time. The trail is the truth; the index is a cache/projection of it.
• Never a snapshot-as-self (Fork-8): the index must not be writable-as-identity; a mutation to "who I am now" is a new era on the trail, then a re-hydration — never an in-place edit of the index.

Acceptance Criteria

• A hydration function produces the current-state index from an IdentityState + its EmbodiedEpisode trail (head era + anchor + social layer).
• The index is deterministically regenerable — same trail → same index; discarding + rebuilding the index yields an identical result (unit-covered).
• There is no write path that mutates the index as identity — state changes append an era + re-hydrate (Fork-8 guard, unit-covered).
• Cites ADR-0032 (re-review trigger e).

Contract Ledger

Notes

• Leaf, one-PR-deliverable (Resolves this); parent epic #14677 (Refs). Depends on #14693 (node-types).
• Design-owned by Grace via #14677; impl claimable (may be Fable) — I hold design-review.
• Provenance: ADR-0032 §2.2.3 (the Fork-8 avoidance clause).