ci: fix nightly workflow (4 longstanding failures)

The nightly workflow has failed every run since it was added. Fix the
four distinct issues so it can actually exercise the slow checks:

- cargo-mutants: drop bogus `--in-diff-base ""` flag (never existed in
  cargo-mutants; v27 rejects it).
- fuzz smoke: pin `--target x86_64-unknown-linux-gnu`. Binstall ships a
  musl-static cargo-fuzz binary whose default target inherits its own
  triple, and libfuzzer's sanitizer is incompatible with `crt-static`.
- miri: move multi-module filters after `--`. `cargo test` only accepts
  one TESTNAME positional; libtest accepts multiple substring filters.
- stateright random_large: add `target_state_count(1_000_000)` so the
  simulation actually terminates instead of looping past the 30 min CI
  timeout. The comment already claimed "up to 10⁶ states".

Also refreshes fuzz/Cargo.lock, regenerated by the local verification
build (was stale vs. parent deps).

tests/stateright_model.rs

@@ -278,6 +278,7 @@ fn random_large() {
         .checker()
         .threads(num_cpus())
         .target_max_depth(50)
+        .target_state_count(1_000_000)
         .spawn_simulation(0, stateright::UniformChooser)
         .join()
         .assert_properties();