test(mutants): add cargo-mutants config for advisory mutation testing

Adds `.cargo/mutants.toml` configuring cargo-mutants to focus on the
algorithmic core (skipping benches, examples, tests_support) and pass
`--test-tool=cargo --no-shuffle` so a mutation that breaks the suite
gets caught quickly.

Mutation testing modifies the source one operator at a time (`>` →
`>=`, `+` → `-`, `true` → `false`, etc.) and re-runs the test suite.
A mutation that *survives* (tests still pass) is a hint that the test
suite isn't checking that bit of behavior — usually because:
- The mutated branch is dead code
- The unit tests rely on side-effects rather than return values
- A property test or invariant is missing

Not wired into CI as a gating check (it's slow — every mutation
re-runs the whole suite). Run locally with `cargo install cargo-mutants`
followed by `cargo mutants --in-diff HEAD~1` for incremental coverage,
or `cargo mutants` for a full sweep.

The config exclusions list explains *why* each module is skipped — most
are the "obvious" kind (benchmark harness, example problems) where
mutation kills are not informative.

.cargo/mutants.toml

@@ -0,0 +1,33 @@
+# cargo-mutants configuration for heuropt.
+#
+# Run with:
+#   cargo install cargo-mutants
+#   cargo mutants                          # full sweep (slow)
+#   cargo mutants --in-diff HEAD~1         # only mutate recently-changed lines
+#
+# A *surviving* mutation = the test suite passed despite a code change,
+# which usually means a missing test or a missing invariant.
+#
+# This isn't gated CI; it's an advisory tool. The property tests in
+# tests/properties.rs are the natural place to land new invariants
+# discovered via mutation runs.
+
+# Files to skip mutating. We skip:
+#   - examples (illustrative, not core algorithm correctness)
+#   - benches (microbench harness, not behavior)
+#   - the docs/* spec markdown
+#   - tests_support (test helpers; mutating them changes test inputs,
+#     not behavior under test)
+exclude_globs = [
+    "examples/**/*.rs",
+    "benches/**/*.rs",
+    "src/tests_support/**/*.rs",
+]
+
+# `cargo-mutants` defaults to `cargo test` for the suite. Keep that.
+# `--no-shuffle` makes failure attribution deterministic.
+additional_cargo_test_args = ["--", "--test-threads=1"]
+
+# Time-out per mutated build+test cycle. Big enough for a slow test
+# (proptest can take ~10s) but short enough to detect infinite loops.
+timeout_multiplier = 5.0