feat: harden shared platform contracts

docs/MODULE_ARCHITECTURE.md

@@ -277,6 +277,23 @@ unsafe methods.
 This avoids retransmitting unchanged snapshots. It does not identify which row
 changed inside a collection.
 
+### Mutation Preconditions
+
+Weak response ETags are cache validators only. Mutable aggregates expose a
+separate positive, monotonic revision and an opaque strong ETag generated by
+`govoplan_core.core.concurrency.strong_resource_etag`. HTTP mutations send that
+strong ETag in `If-Match`; capability and worker calls carry the equivalent
+typed `expected_revision`.
+
+Core's compare-and-set primitive advances the revision in the same transaction
+as the domain mutation. A missing HTTP precondition is `428 Precondition
+Required`, a stale HTTP precondition is `412 Precondition Failed`, and a
+domain/reconciliation conflict is `409 Conflict`. Conflict responses contain
+bounded resource and revision metadata rather than the complete current
+object. Modules may opt into the conservative three-way merge helper, but must
+declare protected workflow, delivery, ownership, lock, evidence, signature,
+and cryptographic paths that can never be merged automatically.
+
 ### Delta Collections
 
 Collection endpoints that can expose row-level changes should use the shared