feat: implement procedure party registry

docs/PARTIES_DOMAIN.md

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+# Parties And Representation Domain
+
+## Ownership
+
+Parties owns procedure-local participation and representation authority. A
+Party references, but does not copy, an Identity, Organization, group, or
+external subject. It belongs to an exact case, workflow, or decision context.
+
+Contact delivery uses frozen snapshot references so later address changes do
+not rewrite evidence. Preferred channels must be permitted by the procedure.
+
+## Representation
+
+Representation powers identify representative and represented Parties, an
+external or internal power reference, permitted actions, effective time, and
+evidence. Existing powers cannot be removed or overwritten. They remain in the
+Party revision and are ended through an explicit, OCC-guarded revocation
+revision. Expired or revoked powers are not effective for downstream delivery.
+
+## Revision And Recovery
+
+Party writes are append-only, replay-safe, tenant-bound, and protected by
+optimistic concurrency. The procedure identity cannot change across revisions.
+Database restore is the recovery unit; downstream effects preserve exact Party,
+contact-snapshot, and representation evidence references.