ADR-012: forked-substrate design — contract-blind boundary, fidelity posture, port deltas

Follow-through on OQ-06/ADR-011: pin the fork's structural decisions
(alkstore-substrate as a vendored path-dep crate, contract-blind API
boundary with contract formulas computed engine-side and pinned
equivalent by the contract suite, keep-the-kept-half API fidelity for
cheap cherry-picks, the W-1/W-2/dead-man's-switch/W-4 port deltas
decided per item, bootstrap re-keying off error-string matching, no
rename migration, deliberate upstream tracking).

Consistency sweep across the doc set for the fork: annotate ADR-003/
005/009/010 and core-contract for superseded ownership facts, fix
schedule-storage table naming (ADR-009 §5, queues.md), re-key ADR-010
§6's notifications hygiene to the at-attach cap the fork scope
realizes, add OQ-11 (scaffold-time residue), and complete both ADR
indexes. Independent review: 0 critical, warnings addressed.

docs/architecture/decisions/004-postgres-driver.md

@@ -101,4 +101,8 @@ Structural constraints the engine owns (all test-pinned in POC #2):
   decision applies per-subsystem.
 - [ADR-006](006-wake-and-delivery-contract.md) — the wake contract the
   forwarder implements on this engine.
-- [engine-postgres.md](../engine-postgres.md) — the engine spec.
+- [engine-postgres.md](../engine-postgres.md) — the engine spec.
+[ADR-003]: 003-sqlite-driver.md
+[ADR-005]: 005-dependency-ownership.md
+[ADR-006]: 006-wake-and-delivery-contract.md
+[queues.md]: ../queues.md