mavend: centralize action validation boundary (slice 4)

docs/design.md

@@ -986,6 +986,48 @@ ends with `make test` green (gofmt + vet + `-race`), no exceptions.
 
 ---
 
+## Prior art — external memory systems
+
+Read before proposing a change to how memory is extracted or read back. Nothing
+here is adopted. Each entry says what does not transfer and what a cheap
+experiment against it would be.
+
+### VoiceMem (github.com/xzf-thu/VoiceMem, Apache 2.0)
+
+A streaming memory system for voice assistants, surveyed 2026-08-30. Python,
+Chinese-first. A "left brain" of keyed structured facts and a "right brain" of
+affect and relationship nodes, both extracted and queried while the user is
+still speaking.
+
+**What does not transfer.** Its speech stack is Paraformer-zh streaming STT with
+Qwen-Omni or Step-Audio2-Mini as the conversational model. Maven runs
+whisper.cpp, piper and Qwen3-1.7B. It is a Python library, so adopting the code
+means a Python service on homesrv behind a new daemon seam (`docs/offload.md`),
+which is a large cost for a Chinese-tuned pipeline. The licence is not the
+barrier.
+
+**What is worth taking.**
+
+- **Streaming extraction.** Extraction and retrieval start on partial
+  transcripts, not on a final one. Maven's `runTurn` waits for mavsttd to
+  finish. This is the larger win and the larger change, because it touches both
+  the STT seam and the turn ladder.
+- **A hard retrieval token budget.** They report roughly 430 memory tokens per
+  query. At `n_ctx` 4096 the constraint binds directly on what memory may put in
+  front of the resident model. This is the cheapest experiment: measure Maven's
+  existing recall evals against a capped budget.
+- **The fact/affect split.** The factual half is what Maven already has. Affect
+  and relationship nodes have no equivalent here, and they bear on § save-where.
+- **A shared embedder.** They also use multilingual-E5, so their retrieval
+  scoring ports without a model change.
+
+**Read their numbers carefully.** 91.2% on LoCoMo against 61.68% for Mem0 is
+self-reported by the authors with no independent replication found. The 134ms
+figure is memory-system latency, not a turn including STT and the resident
+model. Both LoCoMo and PersonaMem are English and Chinese, so no claim there
+holds for Russian recall until a translated fixture exists. Any number taken
+from this section into Maven's prose needs a `docs/evals/` file behind it.
+
 ## Open questions
 
 Router+invocation, two-memory routing, presence, and auth were once listed
@@ -1019,7 +1061,7 @@ here and are resolved by the sections above.
 - **compound captures** — "slept 6h, fan noise wrecked it" = one fact + one
   note in one utterance. Needs a second pass or it loses half.
 - **query read-path** — semantic RAG vs a structured read, depending on the
-  ask.
+  ask. Prior art in § Prior art — external memory systems (VoiceMem).
 - **presence — away tap override** — an explicit `away` tap as a hard
   override. Clean extension, deferred; scoring stands without it.
 - **presence — weights/τ hand-tuning** — first-guess numbers; expect tuning