store: tests for mutation survivors; drop pre-checks the rehearsal covers

From the first store mutation run (324 caught of 396 viable):
- receive_payment and pay_bill repeated the engine's refusals by hand
  after rehearsing the engine itself, so those checks could never fire
  (their mutants survived). Removed: the rehearsal is the check.
- New hand-computed tests: a disposal's final catch-up after some
  depreciation was posted (200 of 300, gain 100); a disposal on the
  purchase date (allowed) or the day before (refused); a retained
  earnings close with a sweep replays it; a float inside an array is
  refused; the viewer's journal and head hash.
- The concurrency test's reader stops when the writer thread ends, not
  only when it says it's done, so a mutant that kills the writer fails
  the test at once instead of spinning to the five-minute timeout.

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

crates/ledger-store/tests/concurrency.rs

@@ -79,7 +79,8 @@ fn a_viewer_reading_while_the_books_are_written_sees_only_whole_books() {
     start.wait();
     let (mut last_seq, mut last_posted, mut refreshes) = (0u64, 0usize, 0u64);
     loop {
-        let finished = done.load(Ordering::SeqCst);
+        // Done, or the writer died (its panic surfaces at the join below).
+        let finished = done.load(Ordering::SeqCst) || writer.is_finished();
         snapshot.refresh().unwrap();
         refreshes += 1;
         let l = snapshot.ledger();