feat(memory): define external adapter boundaries

docs/memory-graph-runtime.md

@@ -46,6 +46,10 @@ with `markitect-tool`.
 - `MemoryRuntimeService.export_runtime_envelope()` emits a portable runtime
   envelope containing graph nodes, edges, memory events, and audit traces with
   operation id, actor, policy decision, and filter metadata.
+- `MemoryRuntimeAdapterCapability`, `MemorySemanticIndex`,
+  `MemoryAuditPublisher`, and `MemoryRuntimeRegistry` define optional adapter
+  boundaries for vector/semantic indexes, durable audit sinks, remote registries,
+  and sibling memory runtimes.
 
 ## Boundary
 
@@ -64,5 +68,24 @@ with `markitect-tool`.
 - retention, refresh, compaction, review gates, and audit behavior
 - agent-safe update plans and Markitect-compatible export envelopes
 
+## External Adapter Boundaries
+
+External graph databases should implement `MemoryGraphRepository`; they are
+storage adapters, not alternate memory domain models. Enterprise policy systems
+should implement `PolicyGateway`; policy decisions remain explicit inputs to
+runtime operations.
+
+Optional advanced adapters use small dedicated ports:
+
+- `MemorySemanticIndex` for vector or hybrid retrieval indexes.
+- `MemoryAuditPublisher` for durable event sinks, webhooks, or SIEM pipelines.
+- `MemoryRuntimeRegistry` for remote memory registries and sibling runtimes
+  such as future phase-memory services.
+
+Adapters advertise `MemoryRuntimeAdapterCapability` with adapter kind,
+supported operations, optional dependencies, and whether they are deterministic
+local adapters. Adapter payloads must be serializable dictionaries or core
+runtime records, so core tests can stay deterministic and dependency-free.
+
 `infospace-bench` should consume these records and Markitect fixtures to measure
 retrieval quality, latency, budget pressure, and regression behavior.