fix(blur): fold the heid bug-hunt: two file names, a reader-judged writer, one predicate

The heid bug-hunt panel on 4cfbce5 (hulda, regin, kimi; groa timed out) found
four real defects in the round-trip fix, and three of its arms converged on the
worst: it re-created the bug it existed to fix.

- Two names, never a sniffed file (3/3). JSON went into the OLD `.blurred`, and
  the reader guessed the format from the bytes, so a legacy file whose one line
  is an item named `["a.png"]` read as {"a.png"} and blurred the neighbour. The
  set now lives in `.blurred.json`, JSON only. The legacy `.blurred` is read as
  lines only, and only while `.blurred.json` is absent; the first write retires
  it, after the new file is in place.
- A planted directory is a 409, not a 500 (2/3 plus a third angle, executed by
  the seat). The reader was hardened against it and the writer was not:
  os.replace and unlink raised IsADirectoryError through the route. Now the
  writer is judged by its reader: set_blurred re-reads after writing and raises
  BlurUnwritable unless the set on disk is the set asked for. That one check
  covers a directory at either name, a permission and a race.
- A lone surrogate is dropped on read (hulda, executed). `"\ud800"` is a valid
  JSON string that no filename can produce, and the UTF-8 encode raised on it
  at every later write.
- The writer respects the reader's size cap (2/3). Nothing capped the write,
  and the reader reads an oversized file as EMPTY, which reveals everything.
- One predicate, check_rel, for the route and the CLI (2/3). The CLI's `*..*`
  substring guard refused `a..b.png`, which the route accepts. It also refuses
  an empty path now (regin, kimi), and every item is checked before any is
  written.
- `booth blur` fails closed, with a message and exit 3, when its package is
  missing (kimi), as `link` already does.

Declined, with reasons: the Item positional-constructor break (booth_items is
the only constructor, INV-1), the fdopen fd leak and the short read (not
constructible on a local filesystem, and the `.seen` shape), and
unreadable-reads-as-revealed (blur is cosmetic; the `.seen` posture).
blur_storage.toml: 20/20 proved. One row came back VACUOUS on its first run,
because `set() or X` is X, and was rewritten before counting.

tests/mutations/blur_storage.toml

@@ -33,54 +33,60 @@ label = "a JSON-only reader: every live line-format file un-blurs on deploy"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_the_legacy_line_format_still_reads"
 old = '''
-    return {ln.strip() for ln in text.splitlines() if ln.strip()}'''
+        return {ln.strip() for ln in text.splitlines() if ln.strip()}'''
 new = '''
-    return set()'''
+        return set()'''
 
 [[mutation]]
 label = "a legacy file that is not JSON reads as nothing instead of falling back"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_a_legacy_rel_that_starts_with_a_bracket_still_reads"
 old = '''
-        data = None'''
+        text = raw.decode("utf-8", "surrogateescape")
+        return {ln.strip()'''
 new = '''
-        return set()'''
+        text = raw.decode("utf-8", "surrogateescape")
+        try:
+            json.loads(text)
+        except ValueError:
+            return set()
+        return {ln.strip()'''
 
 [[mutation]]
 label = "a FIFO blocks the read (no O_NONBLOCK)"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_a_fifo_blur_file_does_not_block_the_read"
 old = '''
-        fd = os.open(booth / BLUR_FILE, os.O_RDONLY | os.O_NOFOLLOW | os.O_NONBLOCK)'''
+        fd = os.open(path, os.O_RDONLY | os.O_NOFOLLOW | os.O_NONBLOCK)'''
 new = '''
-        fd = os.open(booth / BLUR_FILE, os.O_RDONLY | os.O_NOFOLLOW)'''
+        fd = os.open(path, os.O_RDONLY | os.O_NOFOLLOW)'''
 
 [[mutation]]
 label = "the read follows a planted symlink (no O_NOFOLLOW)"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_a_symlinked_blur_file_is_not_followed_on_read"
 old = '''
-        fd = os.open(booth / BLUR_FILE, os.O_RDONLY | os.O_NOFOLLOW | os.O_NONBLOCK)'''
+        fd = os.open(path, os.O_RDONLY | os.O_NOFOLLOW | os.O_NONBLOCK)'''
 new = '''
-        fd = os.open(booth / BLUR_FILE, os.O_RDONLY | os.O_NONBLOCK)'''
+        fd = os.open(path, os.O_RDONLY | os.O_NONBLOCK)'''
 
 [[mutation]]
 label = "the write goes through a planted symlink instead of replacing it"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_a_write_replaces_a_planted_symlink_rather_than_writing_through_it"
 old = '''
-        os.replace(tmp, path)'''
+                os.replace(tmp, path)'''
 new = '''
-        path.write_bytes(Path(tmp).read_bytes()); os.unlink(tmp)'''
+                path.write_bytes(Path(tmp).read_bytes()); os.unlink(tmp)'''
 
 [[mutation]]
 label = "the stored order is not the stated one (invariant 6)"
 file = "booth/blur.py"
 test = "tests/test_blur.py::test_the_file_is_a_json_array_in_sorted_order"
 old = '''
-    body = json.dumps(sorted(current), ensure_ascii=False)'''
+        body = json.dumps(sorted(current), ensure_ascii=False)'''
 new = '''
-    body = json.dumps(sorted(current, reverse=True), ensure_ascii=False)'''
+        body = json.dumps(sorted(current, reverse=True), ensure_ascii=False)'''
 
 [[mutation]]
 label = "the CLI ignores the verb: `unblur` blurs"
@@ -92,13 +98,13 @@ new = '''
 on = True'''
 
 [[mutation]]
-label = "the CLI writes past a refused '..' path"
-file = "scripts/booth"
+label = "the CLI writes past a refused '..' path (the shared predicate loses its component check)"
+file = "booth/blur.py"
 test = "tests/test_blur.py::test_the_cli_still_refuses_a_dotdot_path"
 old = '''
-        *..*) echo "refusing path with '..': $item" >&2; exit 2 ;;'''
+    if not rel or rel.startswith("/") or ".." in rel.split("/"):'''
 new = '''
-        *..*) echo "refusing path with '..': $item" >&2 ;;'''
+    if not rel or rel.startswith("/"):'''
 
 [[mutation]]
 label = "the item's own blur is the composed one (booth fog leaks into it)"
@@ -120,3 +126,87 @@ new = '''
     out = []
     read_blurred(child)
     for it in booth_items(child):'''
+
+# ---- the heid bug-hunt on this change (hulda, regin, kimi), folded -------------
+
+[[mutation]]
+label = "the legacy file is sniffed for JSON again (a `[\"a.png\"]` line blurs the neighbour)"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_a_legacy_line_that_is_valid_json_still_reads_as_a_line"
+old = '''
+        text = raw.decode("utf-8", "surrogateescape")
+        return {ln.strip()'''
+new = '''
+        text = raw.decode("utf-8", "surrogateescape")
+        try:
+            d = json.loads(text)
+            if isinstance(d, list):
+                return {r for r in d if isinstance(r, str)}
+        except ValueError:
+            pass
+        return {ln.strip()'''
+
+[[mutation]]
+label = "no postcondition: a planted directory's OSError is swallowed as success"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_a_planted_directory_at_the_blur_file_is_a_refusal_not_a_crash"
+old = '''
+    if read_blurred(booth) != current:'''
+new = '''
+    if False:'''
+
+[[mutation]]
+label = "the route turns a disk-state refusal into a 500"
+file = "booth/app.py"
+test = "tests/test_blur.py::test_the_route_answers_a_planted_directory_with_409"
+old = '''
+            raise HTTPException(status_code=409, detail=str(exc))'''
+new = '''
+            raise'''
+
+[[mutation]]
+label = "a lone surrogate from a planted file reaches the writer"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_a_lone_surrogate_in_the_file_is_skipped_and_writes_still_work"
+old = '''
+    return {r for r in data if isinstance(r, str) and r and _encodable(r)}'''
+new = '''
+    return {r for r in data if isinstance(r, str) and r}'''
+
+[[mutation]]
+label = "the writer writes a set the reader would refuse and read as nothing"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_the_writer_never_writes_a_set_the_reader_would_refuse"
+old = '''
+        if len(body) > BLUR_MAX_BYTES:'''
+new = '''
+        if False:'''
+
+[[mutation]]
+label = "an empty item path is accepted and stored"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_the_cli_refuses_an_empty_item_path_before_writing"
+old = '''
+    if not rel or rel.startswith("/") or ".." in rel.split("/"):'''
+new = '''
+    if rel.startswith("/") or ".." in rel.split("/"):'''
+
+[[mutation]]
+label = "a double dot INSIDE a name is refused (the old `*..*` substring rule)"
+file = "booth/blur.py"
+test = "tests/test_blur.py::test_the_cli_accepts_a_double_dot_inside_a_name"
+old = '''
+    if not rel or rel.startswith("/") or ".." in rel.split("/"):'''
+new = '''
+    if not rel or rel.startswith("/") or ".." in rel:'''
+
+[[mutation]]
+label = "the CLI dies with a traceback when its package is missing"
+file = "scripts/booth"
+test = "tests/test_blur.py::test_the_cli_fails_closed_without_its_package"
+old = '''
+except ImportError as exc:
+    src = os.environ["BOOTH_SRC"]'''
+new = '''
+except ZeroDivisionError as exc:
+    src = os.environ["BOOTH_SRC"]'''