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

alkstore-sqlite/src/substrate/mod.rs

@@ -53,5 +53,8 @@ mod queue_ops;
 mod schema;
 mod watcher;
 
-pub(crate) use schema::{attach_notify, bootstrap_schema, open_conn};
-pub(crate) use watcher::SharedUpdateWatcher;
+pub(crate) use schema::{
+    Error as SchemaError, Readers, Writer, apply_default_pragmas, attach_notify, bootstrap_schema,
+    open_conn, open_conn_bootstrapped,
+};
+pub(crate) use watcher::{DEFAULT_WATCHER_POLL_INTERVAL, SharedUpdateWatcher, WatcherConfig};