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  amendment (pg-lo posture superseded by ADR-009), open-questions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/README.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-10-03
+last_updated: 2026-10-03 (ADR-009 — pg-lo admitted on POC #7)
 ---
 
 # alkblobs — Architecture
@@ -47,6 +47,18 @@ there — not inherited as hedges:
   admission-gated on POC #7). It also adds
   [requirements.md](requirements.md) — the REQ anchor that ends
   per-session re-derivation of consumer facts.
+- **ADR-009** (2026-10-03) admits `pg-lo` — postgres Large Objects as
+  the fs tier's second engine (feature `pg-lo`, default-off) — on
+  POC #7's passed gate (`docs/research/poc-pglo-findings.md`):
+  contract 10/10 exact-count tests, durable put ≈ fs durable put at
+  ≥1 MiB, descriptorless `lo_get` window gets, transactional LO
+  lifecycle (zero crash orphans), companion-table contract authority.
+  REQ-2's fleet consolidation option is now shipped: a fleet node can
+  run postgres-only. Named deltas: cached-get 20–50× behind
+  page-cache fs single-stream (aggregate serving ~700 MB/s at 16
+  readers), catalog space reused-but-never-returned, autovacuum
+  inherited. The admission door (ADR-003, exercised by ADR-007 and
+  ADR-009) has now shipped both its tier-extending engines.
 
 ## Architecture Documents
 
@@ -73,6 +85,7 @@ there — not inherited as hedges:
 | [006](decisions/006-verification-posture-and-transfer-encoding.md) | Whole-blob verification; transfer encoding excluded by scoping | Accepted |
 | [007](decisions/007-two-kv-engines-sqlite-and-postgres.md) | Two kv engines (sqlite + postgres) behind one trait; constructor-selected | Accepted |
 | [008](decisions/008-trait-size-probe-fleet-gc-and-fs-engines.md) | Trait `size` probe; fleet GC (DB-backed pins, advisory-locked sweeper); fs-tier engines | Accepted |
+| [009](decisions/009-pg-lo-fs-tier-engine.md) | pg-lo — postgres Large Objects as the fs tier's second engine (admitted on POC #7) | Accepted |
 
 ## Open Questions
 
@@ -84,8 +97,10 @@ alkfs intake; carried for visibility, gating nothing here). OQ-09
 (second kv engine) resolved by ADR-007. The pg-lo fs-engine question
 resolved to *sequenced work*, not a parked question: REQ-2/ADR-008
 named it the candidate fs-tier engine and POC #7
-(`docs/research/poc-pglo-spec.md`) is its admission evidence — a task,
-not a wait. **Deferral
+(`docs/research/poc-pglo-spec.md`) was its admission evidence — a
+task, not a wait; that task completed 2026-10-03
+(`docs/research/poc-pglo-findings.md`, passed) and ADR-009 shipped
+the engine. **Deferral
 policy (the "Schrödinger's code" rule):** a *decision this crate needs
 before shipping* may not be deferred on a dependency that is itself
 waiting for this crate to exist. Two parking kinds remain legitimate: