docs(architecture): ADR-013 — family composition, reactive boundary, sweep lease; OQ-11 resolved

- No sibling (alkstore) dependency in-core; reactivity is
  deployment-layer composition above the facade — cadence stays
  embedder-owned (ADR-005), with alkstore the named substrate for it;
  in-crate notify/scheduler/queues/invalidation hooks are non-goals.
- Fleet sweeper lock pinned as 'named exclusive sweep lease'
  semantics; shipped provider unchanged (pg advisory clause);
  provider swaps are ADR-007-shaped substitution doors.
- Composition contract for downstream ADRs: co-tenancy of CAS/GC
  state + reactive machinery + consumer/liveness tables on one fleet
  host; no-cross-store-atomicity ruled; compensation rests on
  idempotent re-puts, windowed deletes, sweeps-as-report-data.
- OQ-11 resolved (the register's one live circular hedge): engine
  features pull their own driver clusters — sqlite becomes feature
  'sqlite' (default-on, carries rusqlite); 'kv'/'large' tier features
  retired; 'local' ungated; pg-pure builds are supported,
  non-default. Deciding fact already in-tree (REQ-2/ADR-009).
- requirements.md: UC-4/REQ-5 records the composition facts
  (operator record 2026-10-11, corroborated by alkstore's
  consumer-inventory).
- Publish guard stated pre-code: facade ships before publish;
  semver-is-contract from first publish (alkstore ADR-017 discipline).
- Amended accepted ADRs' feature wording (ADR-010 §1, ADR-011 §3/§4,
  ADR-003) with supersession notes; README/overview/open-questions/
  backends/store-api/gc aligned; parked index now empty.

Verification: docs-only round (no Rust surface touched); full-tree
single-value check on the feature graph and sweep-lease wording.

docs/architecture/backends-and-dispatch.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-10-03 (ADR-012 — composite predicate gains the pg-lo same-instance clause; kv-only rejection is put-time; GC state hosts on the kv engine; window wording)
+last_updated: 2026-10-11 (ADR-013 — feature graph re-pinned: engine features pull their own drivers; `kv`/`large` tier features retired, `sqlite` default-on, `local` ungated)
 ---
 
 # Backends, tiers, and dispatch
@@ -111,13 +111,20 @@ the store's seams (see "What the Backend trait is").
 
 | Tier | Engine | Feature | Default | Fleet-valid |
 |---|---|---|---|---|
-| kv | sqlite | — (tier default) | yes (tier default-on) | no |
+| kv | sqlite | `sqlite` | **yes (default-on)** | no |
 | kv | postgres | `postgres` | no | **yes** |
 | kv | mem | `mem` | no | no (test/reference engine) |
-| large | local | — (tier default) | yes (tier default-on) | shared media only, explicit assertion |
+| large | local | *(ungated — dependency-free)* | always present | shared media only, explicit assertion |
 | large | pg-lo | `pg-lo` | no | **yes** |
 
-### kv tier (tier default-on, feature `kv`); engines sqlite (default) + postgres (feature `postgres`, default-off) + mem (feature `mem`, default-off)
+Features gate engines, each pulling only its own driver cluster
+(ADR-010 §1 extended to every engine — ADR-013 §4); tiers are
+constructor composition, not features. The tier table's "Feature"
+column is therefore the engine features: `sqlite` default-on carries
+rusqlite (the batteries-included default); a pg-pure build (`sqlite`
+off) is supported, non-default — REQ-2's consolidation posture.
+
+### kv tier (ADR-003/007/011); engines sqlite (feature `sqlite`, default-on) + postgres (feature `postgres`, default-off) + mem (feature `mem`, default-off)
 
 Small blobs — **`kv` is the tier name; the engine is a per-node
 constructor choice** between the shipped engines (ADR-007; ADR-003
@@ -177,7 +184,7 @@ fleet-valid, and the **contract-reference engine**: every other
 engine's conformance tests mirror its suite (the role it already
 played in POC #1's miniature).
 
-### large tier (tier default-on, feature `large`; the tier formerly named `fs` — renamed in ADR-011); engines `local` (default) + `pg-lo` (feature `pg-lo`, default-off)
+### large tier (ADR-003/008/011; the tier formerly named `fs` — renamed in ADR-011); engines `local` (ungated, always present) + `pg-lo` (feature `pg-lo`, default-off)
 
 Large blobs. Like the kv tier (ADR-007), the large tier is **one
 contract, multiple engines** — the medium is a per-node constructor
@@ -301,6 +308,7 @@ alkfs intake may name needs externally; OQ-08).
 | [010](decisions/010-backend-io-seams-and-fleet-state-home.md) | I/O seams + GC-state home | read cursor / staged put; store-core-owned GC state via the engine-state seam; the kv fleet-validity rule |
 | [011](decisions/011-vocabulary-tiers-engines-and-fleet-state.md) | Vocabulary | tier/engine/instance/node/fleet; mem is a kv engine; fs → large; constructor modes |
 | [012](decisions/012-pre-decomposition-consistency-rulings.md) | Fleet activation & GC-state host | `fleet:` constructor declaration; all fleet GC state on the kv engine; pg-lo same-instance clause; kv-only put-time rejection |
+| [013](decisions/013-family-composition-reactive-boundary-and-feature-graph.md) | Feature graph + fleet lease | engine features pull their own drivers (`kv`/`large` tier features retired); sweep lease semantics; no sibling dependency in-core |
 
 ## Open Questions
 
@@ -325,6 +333,8 @@ alkfs intake may name needs externally; OQ-08).
   not adopted (threshold dispatch chose the simpler policy;
   ADR-003 §Context)
 - ADR-003, ADR-004, ADR-005; ADR-012 (fleet activation, GC-state
-  host, the composite predicate's same-instance clause); [store-api.md](store-api.md) (the invariants
+  host, the composite predicate's same-instance clause); ADR-013 (the
+  feature graph's engine-feature ruling, the sweep lease, the family
+  composition contract); [store-api.md](store-api.md) (the invariants
   engines must satisfy); [requirements.md](requirements.md) (the pinned
   vocabulary)