fix(evidence): carry the real index mtime onto gstack-wtree's temp copy

The stat-cache seed (cp of the real index) stamped the temp index "now",
which defeats git's racy-git protection: an entry is only re-hashed when
its cached mtime is not older than the index file itself, so a same-size
rewrite landing in the same second as the last real index write looked
non-racy, kept its stale stat-cache entry, and vanished from the
fingerprint — evidence stayed FRESH after a source change. This is the
CI flake in test/evidence.test.ts "allow-paths carve-out" (sub-second
alignment on fast runners: expected STALE exit 1, got FRESH exit 0).

touch -r restores the original index timestamp, reinstating the exact
racy window git itself uses. Deterministic regression pin in
test/review-log.test.ts reproduces the miss with pinned zero-nsec
timestamps (fails on the old script, passes now); receipts: manual
probe shows the fresh-stamped copy returning the clean tree for a
same-size 'hello'→'howdy' rewrite while the mtime-carried copy detects
it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/gstack-wtree

@@ -42,7 +42,13 @@ case "$REAL_INDEX" in
   *) REAL_INDEX="$TOP/$REAL_INDEX" ;;
 esac
 if [ -n "$REAL_INDEX" ] && [ -f "$REAL_INDEX" ] && cp "$REAL_INDEX" "$TMPIDX" 2>/dev/null; then
-  : # stat-cache-preserving seed
+  # Carry the real index's mtime onto the copy. Git's racy-git protection
+  # re-hashes any entry whose cached mtime is not older than the index file
+  # itself; `cp` stamps the copy "now", which silently marks every entry
+  # non-racy and lets a same-size rewrite in the same second as the original
+  # `git add` keep its stale stat-cache entry — the content change vanishes
+  # from the fingerprint. touch -r restores the original racy window.
+  touch -r "$REAL_INDEX" "$TMPIDX" 2>/dev/null || true
 else
   git -C "$TOP" read-tree HEAD 2>/dev/null || exit 1
 fi

test/review-log.test.ts

@@ -182,6 +182,35 @@ describe('gstack-wtree', () => {
     });
   });
 
+  test('racy-git window: a same-size rewrite pinned to the index timestamp changes the fingerprint', () => {
+    withScratchRepo((repoDir, wtree) => {
+      const file = path.join(repoDir, 'a.txt');
+      const indexPath = path.join(repoDir, '.git', 'index');
+      // ctime can't be restored after a rewrite; production hits this window
+      // when everything lands in the same second (ctime SECONDS match).
+      // trustctime=false isolates the racy mechanism deterministically
+      // instead of racing a second boundary.
+      gitIn(repoDir, 'config core.trustctime false');
+      // Pin the cached entry's mtime to a fixed timestamp (zero nsec, so the
+      // restore below is exact even on USE_NSEC git builds).
+      const pinned = new Date('2026-01-01T12:00:00Z');
+      fs.utimesSync(file, pinned, pinned);
+      gitIn(repoDir, 'add a.txt');
+      const clean = wtree();
+      // Same-size rewrite restored to the pinned stat, with the index file
+      // itself pinned to the SAME timestamp: the entry is stat-identical to
+      // its stale cache and sits exactly on git's racy-git boundary.
+      // gstack-wtree must carry the real index's mtime onto its temp copy —
+      // a fresh-stamped copy marks the entry non-racy, trusts the stale stat
+      // cache, and the edit vanishes from the fingerprint (evidence would
+      // stay FRESH after a source change).
+      fs.writeFileSync(file, 'howdy\n'); // same byte length as 'hello\n'
+      fs.utimesSync(file, pinned, pinned);
+      fs.utimesSync(indexPath, pinned, pinned);
+      expect(wtree()).not.toBe(clean);
+    });
+  });
+
   test('exits non-zero outside a git repo', () => {
     const nonGit = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-wtree-nongit-'));
     try {