test(wallet): pin the default rolling window and the multi-recipient render

Two cargo-mutants survivors, both properties nothing asserted:

- DEFAULT_PERIOD_SECONDS: `24 * 60 * 60` reads as self-evidently a day, which is exactly
  why nothing checked it — any arithmetic slip yields a plausible-looking window while a
  user told "a daily cap" gets some other period. The literal 86_400 is now pinned.
- SpendSummary's Display separator: every single-recipient fixture makes the "is this the
  first?" test true every time, so `>=`, `<` and `==` all rendered identically. A
  two-recipient fixture asserting the whole line can see it.

Also untrack cargo-mutants scratch output and gitignore it.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MichaelTaylor3d

.gitignore

@@ -1 +1,4 @@
 target/
+
+# cargo-mutants scratch output (regenerable; a full run is minutes of CPU, never a build input)
+mutants.out*/