docs(architecture): ADR-009 — pg-lo admitted as the fs tier's second engine on POC #7

- ADR-009 (new): postgres Large Objects ship as the fs tier's second
  engine (feature pg-lo, default-off), admitted on POC #7's passed
  gate (contract 10/10, durable put ~= fs at >=1 MiB, lo_get window
  gets, transactional LO lifecycle = zero crash orphans). Engine
  shape: companion table (lo_entries) is the contract authority; put =
  one tx (lo_create + lowrite + row + pin); get = descriptorless
  lo_get windows (lo_*64 discipline); named operator deltas (catalog
  space never returned / autovacuum inherited / sweep for the legacy
  bypass class only) and named performance deltas (cached-get 20-50x
  behind page-cache fs single-stream, ~700 MB/s aggregate at 16
  readers = the fleet serving picture).
- Ripple: backends-and-dispatch (pg-lo shipped; fleet locality
  contract gains the whole-fleet-pg-lo option), overview (layer map,
  feature table + pg-lo row, OQ -> shipped pointer), requirements
  (engine def + REQ-2 note updated), README (current-state + ADR
  table), store-api (invariant 1/6 engine lists), ADR-008 Status
  amendment (pg-lo posture superseded by ADR-009), open-questions
  (audit trail completed: decided -> sequenced -> completed).

Verification: docs-only; all cross-references resolve (ADR/REQ/POC);
register, findings, spec, and both engine-ADR status lines agree.

docs/architecture/requirements.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-10-03
+last_updated: 2026-10-03 (pg-lo admitted by ADR-009 on POC #7 — REQ-2's consolidation option is shipped)
 ---
 
 # Requirements
@@ -16,8 +16,7 @@ the ADRs re-derive from it.**
 
 - **Engine** — a concrete storage implementation of one tier's
   contract: the kv tier's engines are sqlite and postgres (ADR-007);
-  the fs tier's engines are `local` (default) and `pg-lo` (candidate —
-  see REQ-2, ADR-008).
+  the fs tier's engines are `local` (default) and `pg-lo` (ADR-009).
 - **Pool** — the flat content-addressed namespace of one deployment
   (`hash → bytes` everywhere; ADR-005).
 - **Node** — one running store instance (one process embedder; its fs
@@ -62,7 +61,8 @@ replicators; ACL/ownership above (alkcall layer).
   Consequences are derived in ADR-008: fleet GC (DB-backed pins,
   advisory-locked sweeper, SQL delete arbitration), the fs-tier
   engine concept with its locality contract, and the pg-lo candidate
-  engine (POC #7).
+  engine (POC #7 — **passed** 2026-10-03, admitted as a shipped
+  engine by ADR-009: a fleet node can consolidate on postgres-only).
 - **REQ-3 (traces: ADR-007, POC #5 B3/B4)** — concurrency scale-out:
   the fleet's hub-class nodes need put throughput that *increases*
   under concurrency (pg: near-linear to ~37k puts/s at 24 conns);