Keep taking messages while a peer's session goes down (closes #216)

The table's PeerDown visits every prefix under the write lock, which
takes over a second on a full table. The prefix handler called it
inline, so its queue filled and the streamer dropped updates.

PeerDown now lets the lock go after every 10,000 prefixes. The handler
runs it on a goroutine of its own, one peer at a time, and holds back
the down peer's later messages until it returns, then applies them in
order. Stop waits for this before its last flush, and logs the
held-back messages it could not apply.

Model: opus-5-5

TODO.md

@@ -35,6 +35,13 @@ tested.
 
 # Completed Steps
 
+- 2026-11-02: a peer's session going down no longer stops the prefix handler.
+  The handler runs the table's `PeerDown` on a goroutine of its own, one peer at
+  a time, and goes on taking messages. It holds back the down peer's later
+  messages until its `PeerDown` returns, then applies them in order. `PeerDown`
+  lets the table's lock go after every 10,000 prefixes, so the IP lookup and the
+  pages no longer wait for the whole visit. A stop past its deadline logs the
+  held-back messages it did not apply (closes #216)
 - 2026-10-27: the tests of `internal/routewatch` no longer fail on a loaded
   host. Five of them gave `HandleMessage` two seconds to apply a full prefix
   batch, which under the race detector can take longer. A test now waits up to