test(r2): commit the round's falsifiers as a mutation table; one flag predicate

tests/mutations/r2_flow.toml: 18 falsifiers, each proved RED under its
change by scripts/mutation_check.py (18/18). Its first run found three
vacuous proofs, now resolved:
- landed_at's per-entry skip: the symlink-loop fixture stopped raising
  once the clock moved to lstat. New fixture: a folder that lists but
  cannot be searched.
- the Desk's bench URL guard: the test covered bookmarks only. A
  hand-edited registry bench now rides with it.
- flagged_targets' `error is None`: defence in depth (hydration already
  strips a damaged mark's target), so no single-guard row; named in the
  table header instead.

The rail's flagged filter and the orphan-flag list read flagged_targets
rather than restating it; no reachable behaviour changes.

booth/app.py

@@ -1206,8 +1206,8 @@ def create_app(
                 # no tray slot, so it is listed apart with its withdraw control
                 # rather than vanishing from the page while staying in the file.
                 "orphan_flags": [m for m in marks
-                                 if m.shape == "flag" and m.error is None and m.target
-                                 and m.target not in {it["name"] for it in gallery}],
+                                 if m.shape == "flag" and m.target in
+                                 flagged_targets(marks) - {it["name"] for it in gallery}],
                 "ord_width": len(str(len(gallery))),
                 "uploaded": (booth / UPLOAD_MARKER).exists(),
                 # The same provenance line the index card carries. Deliberate:
@@ -1238,11 +1238,12 @@ def create_app(
         """
         active = requested if requested in FILTERS else "all"
         open_ids = {m.id for m in open_marks(marks)}
+        flagged = flagged_targets(marks)          # THE flag predicate (R2)
         buckets: dict[str, list[dict]] = {f: [] for f in FILTERS}
         for it in gallery:
             mine = marks_for_target(marks, it["name"])
             buckets["all"].append(it)
-            if any(m.shape == "flag" and m.flagged for m in mine):
+            if it["name"] in flagged:
                 buckets["flagged"].append(it)
             if any(m.shape == "note" for m in mine):
                 buckets["annotated"].append(it)