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  amendment (pg-lo posture superseded by ADR-009), open-questions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

@@ -129,7 +129,8 @@ lesson generalizes to any kv engine).
 
 1. Whole-blob put/get round-trips byte-identical under the canonical
    derivation, both tiers, both put paths, **and every shipped engine
-   of both tiers** (kv: sqlite + postgres; fs: `local`; ADR-007/008)
+   of both tiers** (kv: sqlite + postgres; fs: `local` + pg-lo;
+   ADR-007/008/009)
    (interop with real git remains the source of truth, per POC #1's
    lesson about hardcoded vectors).
 2. Dedup: putting identical content twice (same or different path) is
@@ -147,8 +148,8 @@ lesson generalizes to any kv engine).
    tests assert through sweep outcomes (POC #1 finding 2's lesson
    codified).
 6. `size`/`stat` on any engine returns the entry length without
-   fetching content (asserted per-backend: kv probe, fs stat —
-   ADR-008).
+   fetching content (asserted per-backend: kv probe, fs stat, pg-lo
+   companion-row probe — ADR-008/009).
 
 ## Design Decisions