SQLite engine open: SqliteOpts, connection architecture, spawn_blocking seam posture (task sqlite-engine-open-opts)

- SqliteOpts (poll_interval: Option<Duration>, None = 1 ms shipping
  default ADR-023 §4; max_readers, DEFAULT_MAX_READERS = 8; opts
  exemption from non_exhaustive per ADR-017 §3)
- open(path, opts) -> Box<dyn Store>: writer slot, reader pool,
  SharedUpdateWatcher with fallible spawn mapped W-2-style into
  Error::Database; plain-path posture (ADR-023 §3) pinned by test
- Substrate delta D-30: open_conn_bootstrapped — every connection
  (writer and pooled readers) carries the full bootstrap surface
  (pragmas, notify, alkstore functions, schema) per engine-sqlite.md;
  lineage opened readers pragmas-only. Registered in PROVENANCE.md
- spawn_blocking seam helper + error mappings (string/rusqlite ->
  Database, source chains preserved); helper cfg(test)-gated until
  sqlite-engine-seam-tx wires the trait impls
- Store trait stubbed with Database errors (no panics); close()/Drop
  join the watcher, clear subscribers (death-guard), close pool+writer
- 15 new tests (open boot, watcher fanout/death-close, cadence
  accounting, plain-path literal filename, :memory:, pool bounding,
  drop teardown, seam smoke incl. panic mapping); gates green

Cargo.lock

@@ -43,6 +43,7 @@ dependencies = [
  "rusqlite",
  "serde_json",
  "thiserror",
+ "tokio",
 ]
 
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