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