store: set aside a torn last journal line on open; server waits for lock

A crash or power loss mid-append can leave a partial last line. The
writer acknowledges an append only after its whole line is synced, so
that line was never acknowledged: opening now saves those bytes to
journal.jsonl.torn.<time>, truncates to the last newline, and journals
journal.tail_repaired (replay and projection ignore it; the GUI shows
it; books.digest reports it in journal_repairs). Damage before the last
newline is still corruption and still refuses to open. Tested at every
byte of the last append.

The MCP server now waits up to ten seconds for the writer lock: a
restarted server (a client reconnecting) raced the exiting one and died
with Locked, which also made mcp_schedules_and_recurring_over_stdio
flaky. The test harness now waits for each server to exit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-mcp/tests/mcp_stdio.rs

@@ -9,10 +9,18 @@ struct Mcp {
     stdin: ChildStdin,
     stdout: BufReader<std::process::ChildStdout>,
     next_id: u64,
-    #[allow(dead_code)]
     child: Child,
 }
 
+/// Closing stdin ends the server; wait for it to exit, so the next server
+/// on the same books never races it for the writer lock.
+impl Drop for Mcp {
+    fn drop(&mut self) {
+        let _ = self.child.kill();
+        let _ = self.child.wait();
+    }
+}
+
 impl Mcp {
     fn spawn(data_dir: &std::path::Path) -> Self {
         Self::spawn_with(data_dir, &[])
@@ -1673,3 +1681,29 @@ fn mcp_receipts_always_required_when_threshold_is_zero() {
         "a $5 entry needs a receipt too: {err}"
     );
 }
+
+/// A client reconnecting starts a new server while the old one still has
+/// the books: the new one waits for the lock instead of dying.
+#[test]
+fn a_restarted_server_waits_for_the_old_one_to_let_go() {
+    let dir = test_dir("restart-race");
+    let init = |mcp: &mut Mcp| {
+        mcp.request(
+            "initialize",
+            json!({
+                "protocolVersion": "2025-06-18",
+                "capabilities": {},
+                "clientInfo": {"name": "smoke-test", "version": "0.1.0"},
+            }),
+        );
+        mcp.notify("notifications/initialized");
+    };
+    let mut old = Mcp::spawn(&dir);
+    init(&mut old);
+    let mut new = Mcp::spawn(&dir);
+    std::thread::sleep(std::time::Duration::from_millis(500));
+    drop(old);
+    init(&mut new);
+    let chart = new.call("books.chart", json!({}));
+    assert!(chart.is_object());
+}