docs: the pg LISTEN identifier-truncation wake-collision note (review 004 item 13)

deployment.md's Postgres co-tenancy paragraph and engine-postgres.md's
connection-architecture wake-substrate section now carry the 63-byte
identifier-truncation collision note as dated annotations: two names
longer than 63 bytes sharing a 63-byte prefix silently register the
same server-side wake channel; pg_notify (the parameter path, untrun-
cated, rejecting >= 64-byte channels) is unaffected, so the collapse is
on the registration side. Mitigation: keep mechanism names at most 63
bytes (byte count). The naming contract's deliberate no-cap verdict is
untouched. Discharges review 004 'findings recorded, not fixed' item
13 (task docs-pg-identifier-truncation-note).

docs/architecture/engine-postgres.md

@@ -1,6 +1,9 @@
 ---
 status: stable
-last_updated: "2026-10-10 (review-wave-5 gate: the verification backlog fully discharged in the version-stamped contract suite, green against this engine; spec flipped draft → stable)"
+last_updated: "2026-10-11 (review 004 item 13's wake-channel
+identifier-truncation note — the doc-fix disposition — added to the
+connection-architecture wake-substrate section).
+Earlier, 2026-10-10 (review-wave-5 gate: the verification backlog fully discharged in the version-stamped contract suite, green against this engine; spec flipped draft → stable)"
 ---
 
 # Postgres engine
@@ -60,6 +63,26 @@ obligations live in the core spec.
   broadcast re-subscribe (no server round-trips); per-channel
   connections are never warranted at this scale (POC-verified).
 
+One wake-channel naming hazard, noted per the review 004 item 13
+disposition *(annotated 2026-10-11)*: the forwarder's
+`LISTEN`/`UNLISTEN` carry the channel as a quoted SQL identifier
+(`quote_ident` — the POC's quoting shape), and PostgreSQL truncates
+identifiers to 63 bytes
+(`NAMEDATALEN` − 1) server-side — with only a `NOTICE`, never an
+error — so two distinct names longer than 63 bytes sharing a
+63-byte prefix register the same server-side wake channel, silently.
+The `pg_notify` path is unaffected by the truncation — it carries
+the channel as a text parameter, not an identifier (channels of 64
+bytes or more are rejected there server-side) — so the collapse is
+on the registration side, not the send side. Names are unbounded by
+the contract's naming rules (deliberate; [ADR-008](decisions/008-contract-v1-pinning.md)
+§4), nothing engine-side warns, and the machinery itself is
+name-length agnostic (63 bytes is a server-side limit, not an
+engine check). Mitigation is a consumer obligation: keep channel
+names under the PostgreSQL identifier limit (at most 63 bytes; the
+limit counts bytes, not characters) — the operator-facing statement
+lives in the [deployment matrix](deployment.md).
+
 ## Mapping the contract
 
 | Contract piece | Engine realization |