merge(r2 follow-up): the EACCES blast radius, and r2's falsifier table

design-dev's two follow-up commits on the R2 branch.

167f265 is PRE-EXISTING and his to have found, not his to have caused:
Path.is_file() swallows ENOENT but PROPAGATES EACCES, so one folder with r--
and no x in one booth made booth_items raise — and list_booths calls it for
every booth, so the index 500s for all of them. Identical blast radius to the
0xff filename the bug-hunt panel found, arriving through a different syscall.

39a3cb2 commits R2's own falsifiers as tests/mutations/r2_flow.toml, 18 rows.
Its first run caught three vacuous proofs, which is the fourth time this week
that running the mutation has disagreed with reading the assertion.

# Conflicts:
#	booth/items.py

booth/app.py

@@ -1229,8 +1229,8 @@ 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:
@@ -1261,11 +1261,12 @@ 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)