fix(scripts): the mutation harness must not churn source mtimes

It rewrites a tracked file and restores it byte-for-byte — but the restore
bumped the mtime, and in this repo that is not cosmetic. The repo IS the
deployment root and nothing takes effect until the service restarts, so 'is
:8090 stale?' is answered by comparing the service's start time against source
mtimes. A tool that moves those without changing a byte makes that check lie:
it reported the live service 16 minutes stale while it was serving current code.

Restores atime/mtime with os.utime, with a test whose defeating change is
dropping that line. Found by using the staleness check for real, not by review.

649 green; 12/12 U7 falsifiers still proved.

tests/test_mutation_check.py

@@ -108,3 +108,23 @@ def test_the_source_is_restored_even_when_the_mutation_proves(tmp_path):
     check({"label": "flip", "file": "mod.py", "test": "test_probe.py::test_f",
            "old": "return 2", "new": "return 3"}, repo=repo)
     assert (repo / "mod.py").read_text() == before
+
+
+def test_a_reverted_file_keeps_its_mtime(tmp_path):
+    """The repo IS its own deployment root: nothing takes effect until the
+    service restarts, so "is :8090 stale?" is answered by comparing the
+    service's start time against source mtimes. A tool that rewrites a file
+    with identical bytes still bumps its mtime and makes that check lie — it
+    reported the live service 16 minutes stale when it was current.
+
+    Defeating change: dropping the os.utime in the restore."""
+    import os
+
+    repo = _tree(tmp_path, "def f():\n    return 2\n",
+                 "def test_f():\n    assert f() == 2\n")
+    mod = repo / "mod.py"
+    os.utime(mod, (1_000_000_000, 1_000_000_000))
+    before = mod.stat().st_mtime_ns
+    check({"label": "flip", "file": "mod.py", "test": "test_probe.py::test_f",
+           "old": "return 2", "new": "return 3"}, repo=repo)
+    assert mod.stat().st_mtime_ns == before