docs: open Phase 1 — architecture spec set over the Phase 0 evidence

docs/architecture/ now exists: README index, overview, five component
specs (core-contract, engine-sqlite, engine-postgres, queues,
deployment), ADR-001..007 carrying the Phase 0 resolved decisions
(crate split, feature scope, per-engine drivers, dependency
ownership, wake contract, tx seam), and the centralized
open-questions tracker promotion: OQ-ST-01..08 mirror to OQ-01..08
one-to-one with statuses/resolutions carried; new Phase 1 questions
append (OQ-09 scheduler collapse, OQ-10 contract versioning).
Open Phase 1 work: OQ-04 contract pinning (high), OQ-05 queue
semantics depth (high), OQ-06 honker-core quality read (high;
fork-trigger gate), OQ-08 capability surface, OQ-09, OQ-10.

Erratum fixed in phase-0 OQ-ST-04 (thread-affinity friction is
SQLite-side, previously garbled as pg-side) and a stale scheduler-
boundary pointer corrected in consumer-inventory.md. Two review
passes run (findings: OQ-promotion numbering faithfulness, ADR
back-reference sync) — all critical/warning findings resolved.

docs/architecture/engine-sqlite.md

@@ -0,0 +1,114 @@
+---
+status: draft
+last_updated: 2026-10-04
+---
+
+# SQLite engine
+
+The `alkstore-sqlite` engine implements
+[core-contract.md](core-contract.md) on rusqlite + published
+honker-core. This spec records WHAT the engine is internally (its
+connection architecture, seam, and wake plumbing) — not code-level
+HOW. Decisions live in ADRs; per-contract obligations live in the core
+spec and are not restated here.
+
+## Identity and posture
+
+- Single driver: rusqlite (riding
+  [honker-core's pin](decisions/003-sqlite-driver.md)),
+  `bundled-sqlite` for hermetic builds. No `.so` runtime artifacts, no
+  vendored patches
+  ([ADR-003](decisions/003-sqlite-driver.md),
+  [ADR-001](decisions/001-crate-split.md)).
+- Sync machinery, async-facing trait: honker-core is sync (std
+  threads, blocking iterators); the engine bridges at the trait seam
+  per the family-standard posture (alktty REQ-TTY-01 precedent).
+  Every engine call runs in `spawn_blocking`.
+- Single-host by nature: file-backed, one machine, NFS-two-writers
+  unsupported (honker's honesty posture, inherited). See
+  [deployment.md](deployment.md).
+
+## Connection architecture
+
+- **Writer** — one dedicated connection; the only one permitted to
+  write. Serializes all mutations (WAL single-writer, modeled
+  honestly, not fought).
+- **Readers** — a small pool of read connections for lookups and
+  claim/ack work.
+- **Watcher** — honker-core's `SharedUpdateWatcher`: a dedicated
+  thread polling `PRAGMA data_version` at the default 1 ms cadence,
+  fanning out to listeners, overtriggering on purpose (waking all
+  subscribers per poll tick, even when several commits coalesced
+  inside one tick — wake is a hint; consumers re-read indexed state,
+  [ADR-006](decisions/006-wake-and-delivery-contract.md)).
+- Each connection runs honker's bootstrap at open: pragmas
+  (WAL, `synchronous=NORMAL`, busy timeout), `attach_notify`,
+  `attach_honker_functions`, `bootstrap_honker_schema` (the
+  alknet-filesystem POC's wiring shape).
+
+## Mapping the contract
+
+| Contract piece | Engine realization |
+|---|---|
+| notify / listen | honker's notify functions inside the caller's tx; `listen()` bridges the watcher's fanout into a tokio receiver (one `spawn_blocking` thread per subscription doing `blocking_send`) |
+| streams | honker's stream machinery; explicit offset saves through the tx seam |
+| queues | honker's queue functions ([ADR-002](decisions/002-feature-scope.md)); semantics depth design in [queues.md](queues.md) |
+| named locks | honker's lock machinery |
+| scheduler / outbox | honker's counterparts, per [queues.md](queues.md) |
+| begin_tx | acquires the writer slot, opens `BEGIN IMMEDIATE`, returns the caller-held handle ([ADR-007](decisions/007-transactional-seam.md)) |
+| handle ops | each `*_tx` op round-trips `spawn_blocking` to the same connection (thread-affinity note in [ADR-007](decisions/007-transactional-seam.md)) |
+| commit/rollback | releases the writer slot |
+
+## Obligations and constraints
+
+- **A long transaction parks the writer** (the slot lease is the
+  honest model). Contract docs must surface this so consumers budget
+  transactions ([ADR-007](decisions/007-transactional-seam.md) negative
+  consequence).
+- Watcher failure handling is inherited: on watcher death, every
+  subscriber's receiver closes (`WatcherDeathGuard` behavior) —
+  consumers see the close, never a silent hang
+  ([ADR-006](decisions/006-wake-and-delivery-contract.md)).
+- Wake coalescing: bursts inside one poll tick produce one wake;
+  correctness is preserved by the re-read contract
+  (POC-pinned: missed-wake stress with correct post-burst re-reads).
+- **Deployment note**: honker-core 0.5 pins rusqlite ^0.40.1, whose
+  rustc floor is ≥ 1.99; binaries linking this engine carry that
+  requirement ([deployment.md](deployment.md) matrix).
+- The optimization path, if seam throughput ever demands it: a
+  dedicated std-thread bridge (one thread owning the writer conn, ops
+  over mpsc — measured ~2× the spawn_blocking shape at p50 in POC #1),
+  or a raised watcher cadence for idle CPU. Neither is the default.
+
+## What rides on the fork question
+
+Nearly all of honker-core's surface is consumed (Writer/Readers/
+SharedUpdateWatcher/attach_*). The [quality read
+(OQ-06)](decisions/005-dependency-ownership.md) is this engine's only
+open dependency gate: if it names a defect or an upstream-unwon't
+change, the fork posture
+([ADR-005](decisions/005-dependency-ownership.md)) fires and this
+engine's substrate becomes owned code. Until then, published-library
+consumption stands.
+
+## Design Decisions
+
+| ADR | Decision | Summary |
+|---|---|---|
+| [001](decisions/001-crate-split.md) | Crate split | single-driver engine crate |
+| [002](decisions/002-feature-scope.md) | Feature scope | which rows this engine serves |
+| [003](decisions/003-sqlite-driver.md) | Driver | rusqlite + honker-core, bridged seam, inherited watcher |
+| [005](decisions/005-dependency-ownership.md) | Ownership | published honker-core; named fork triggers |
+| [006](decisions/006-wake-and-delivery-contract.md) | Wake contract | data_version watcher, coalescing, death-closes-receivers |
+| [007](decisions/007-transactional-seam.md) | Tx seam | writer-slot lease, `BEGIN IMMEDIATE`, `spawn_blocking` round-trips |
+
+## Open Questions
+
+Open questions are tracked in
+[open-questions.md](open-questions.md). Key
+questions affecting this document:
+
+- **OQ-06**: honker-core quality read — fork-trigger assessment (open)
+- **OQ-09**: scheduler collapse into queues (shared with
+  [queues.md](queues.md)) (open)
+- **OQ-05**: queue semantics depth on honker's machinery (open)