sqlite: the listen bridge takes the stream bridge's std-thread shape (review 004 item 8) — one dedicated named thread per subscription doing blocking_send, not a spawn_blocking task (past ~500 listeners the blocking-pool ceiling queued the whole engine's seams); spawn failure unsubscribes and fails closed with Error::Database, matching the stream bridge's posture; notify.rs + engine-sqlite.md docs aligned; task -> completed

alkstore-sqlite/src/notify.rs

@@ -9,9 +9,12 @@
 //!   never produces [`Error::PayloadTooLarge`](alkstore::Error) (ADR-016
 //!   §5); the payload crosses the notify table as the serde_json
 //!   serialization's UTF-8 text, the byte form ADR-020 §4 pins.
-//! - `listen` — the watcher-fanout bridge: one `spawn_blocking` thread
-//!   per subscription (`engine-sqlite.md`'s mapping row) looping
-//!   `recv()` on the substrate's sync feed (1-slot — bursts coalesce)
+//! - `listen` — the watcher-fanout bridge: one dedicated `std::thread`
+//!   per subscription (the stream bridge's aligned shape — review 004
+//!   item 8; not a tokio `spawn_blocking` per subscription, which
+//!   would queue the engine's seams behind parked bridges at tokio's
+//!   blocking-pool ceiling) looping `recv()` on the substrate's sync
+//!   feed (1-slot — bursts coalesce)
 //!   and `blocking_send`-ing [`Wake`] values into a tokio channel the
 //!   [`WakeReceiver`] owns. Wakes are coalesced hints (overtrigger on
 //!   purpose, ADR-006): the watcher fires on *any* commit, so a wake
@@ -88,6 +91,10 @@ pub(crate) fn notify(
 /// against a store whose watcher has *died* (the db file was replaced)
 /// fails closed the same way — a new subscription never enrolls onto a
 /// dead feed to park forever (review 004 item 2's new-subscriber arm).
+/// The bridge runs on a dedicated `std::thread` (the stream bridge's
+/// shape — review 004 item 8); a thread-spawn failure unsubscribes and
+/// fails closed with [`Error::Database`](alkstore::Error) (the stream
+/// bridge's posture).
 pub(crate) fn listen(
     watcher: Arc<SharedUpdateWatcher>,
     closed: Arc<AtomicBool>,
@@ -102,13 +109,21 @@ pub(crate) fn listen(
     let (id, rx) = watcher.subscribe().map_err(database_error)?;
     let (wake_tx, wake_rx) = tokio::sync::mpsc::channel(WAKE_CHANNEL_CAPACITY);
     let channel = channel.to_string();
-    let _bridge = tokio::task::spawn_blocking(move || {
-        while let Ok(()) = rx.recv() {
-            if wake_tx.blocking_send(Wake::new(channel.clone())).is_err() {
-                break;
+    let spawned = std::thread::Builder::new()
+        .name(format!("alkstore-listen-bridge-{channel}"))
+        .spawn(move || {
+            while let Ok(()) = rx.recv() {
+                if wake_tx.blocking_send(Wake::new(channel.clone())).is_err() {
+                    break;
+                }
             }
-        }
-    });
+        });
+    if let Err(e) = spawned {
+        watcher.unsubscribe(id);
+        return Err(Error::database(std::io::Error::other(format!(
+            "failed to spawn listen bridge thread: {e}"
+        ))));
+    }
     Ok(Box::new(SqliteWakeReceiver {
         wake_rx,
         watcher,