fix(603): close four defects found in adversarial review of the stale-after fields

1. `date.fromisoformat` is not a YYYY-MM-DD validator. On Python >= 3.11 it also
   accepts ISO basic format ("20270101") and week dates ("2027-W01-1"), so a
   malformed-looking value passed the blocking check — and which forms parse
   depends on the interpreter, meaning the same corpus could validate differently
   on a dev machine and on the runner (pr-checks.yml pins only python-version
   '3.x'). Knock-on: the catalog's Review-due section sorts on the raw STRING, so
   an accepted "20270101" sorted AFTER "2027-01-15" ('-' < '0'), contradicting the
   section's own "sorted soonest-first" text. Gate on ^\d{4}-\d{2}-\d{2}$ first,
   which fixes both — a fixed-width zero-padded form makes string sort == date sort.

2. A present-but-empty `stale-after:` was collapsed to None by `or None` in the
   parser and then skipped by a truthiness guard in the validator, so it passed as
   "absent" — a field that silently never fires, which is the exact failure mode
   the blocking check exists to prevent. Keep "" distinct from None and test with
   `is not None`.

3. `test_catalog_is_date_independent` was partly vacuous: with no date in either
   render, both sides were trivially equal after the .replace(). It did still catch
   an injected clock-derived marker, but it passed with the feature deleted. Assert
   the dates are present.

4. The malformed-date check ran only over the active set, exempting archive
   records. Staleness is moot there, but a typo is still a typo — check both wings.

Adds regression tests for each, plus a Review-due row for a topic-file record
(pinning the `../decisions.md` vs bare-filename link forms).

scripts/tests/test_decisions_lib.py

@@ -47,3 +47,15 @@ def test_optional_fields_default_to_none_when_absent():
     r = next(r for r in recs if r.key == "ci.runner-placement")
     assert r.stale_after is None
     assert r.sources is None
+
+
+def test_empty_stale_after_parses_as_empty_string_not_none():
+    """Absent vs present-but-empty must stay distinguishable for the validator."""
+    text = (
+        "## H\n"
+        "`key: a.b` · `status: active` · `since: 2026-01-01` · `stale-after:` "
+        "· `supersedes: none` · `superseded-by: none`\n"
+        "**Rule:** r\n"
+    )
+    r = dl.parse_text(text, Path("fake.md"))[0]
+    assert r.stale_after == ""