docs(architecture): ADR-007 — two kv engines (sqlite + postgres) behind one trait

Dissolve OQ-10's circular "standing offer" framing and record the
engine decision the POC evidence already supported:

- ADR-007 (new): kv tier ships sqlite (default) + postgres (feature
  `postgres`, default-off), constructor-selected per node; records the
  deployment economics (classical kv+fs+relational downstream stack
  collapses to one relational engine + fs), the pg impl contract
  (POC #5 B5 posture deltas), the CI sweep-safety gate under
  --all-features, redb ruled out (POC #6), and posture defaults as
  deferred cost in the ADR-006 pattern
- ADR-003: amended status (engine pin widened by ADR-007); kv entry
  notes the widened engine set
- ADR-004: reconciliation note — the "two backends" count is tiers,
  not engines
- backends-and-dispatch.md: tier≠engine distinction, pg engine
  section, co-tenancy note (same-file consumer writes share sqlite's
  single-writer ceiling), substitution-door precedent
- open-questions.md: OQ-10 resolved-by-ADR-007, with the
  why-this-is-not-Schrödinger's-code record (a deployment running
  alkblobs cannot pre-exist the pg engine — OQ-09 precedent); parked
  index narrowed to OQ-07/OQ-08
- overview.md: layer map + dependency posture reflect the `postgres`
  feature; README.md: ADR-007 in tables + corollary applications
- phase-0.md / pg+redb findings: promotion + supersession notes

Trigger framing survives only as deployment guidance (which engine a
node chooses — topology, not throughput), not as a gate on the crate's
own work.

Verified: cargo test, clippy --all-targets -D warnings, fmt --check.
Docs-only change (Phase 1 architecture); no implementation yet.

docs/research/poc-redb-kv-findings.md

@@ -205,11 +205,13 @@ go up.
 - **Reads: no case for redb on merit** — page-cache reads are what any
   of these give the large tier for free; sqlite's 19 µs probe was
   already fine.
-- **ADR action: none.** SQLite's pin now has four-way triangulated
-  evidence (POC #3 fs, POC #4 pg, POC #6 redb, plus the lineage
-  citation). If a future consumer wanted redb-shaped durability
-  trade-offs (ephemeral cache tier), that's a *different* tier than
-  the kv engine slot.
+- **Anti-recommendation adopted (ADR-007, 2026-10-02): redb stays
+  ruled out** — recorded in the decision so the question does not
+  reopen implicitly. SQLite's engine pin now has four-way triangulated
+  evidence (POC #3 fs, POC #4/#5 pg, POC #6 redb, plus the lineage
+  citation), and ADR-007 ships the kv tier as sqlite + postgres. If a
+  future consumer wanted redb-shaped durability trade-offs (ephemeral
+  cache tier), that's a *different* tier than the kv engine slot.
 
 ## POC quality notes