fix(projection): retirement must release the identifier it holds

`slug` is unique across the whole table, so retiring a row by timestamp alone
leaves its identifier locked to a record nothing derives any more — and the
repository that legitimately owns it can never claim it. core-hub's inherited
REPO-WP-0001/0002 rows retired cleanly and repo-seed stayed refused with
"slug already belongs to another repository", pointing at two retired rows.

Retirement now stamps the slug `<slug>@retired-YYYYMMDD`. The row, its UUID and
its progress events are untouched, so history stays attached to the record it
happened under; only the human-facing identifier is freed. Re-retiring an
already-stamped row replaces the mark rather than stacking it, or the 100-char
column overflows after a few passes.

The UUIDs never collided here: core-hub's rows predate derived identity and
carry v4 UUIDs, while repo-seed derives v5 ones. The identifier was the only
thing in the way.

733 pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 2583210@bnt-lap001
Assistant-Session: f2bff2d5-e9b2-4338-92ca-10282a927006

tests/test_forge_projection.py

@@ -752,3 +752,29 @@ class TestRekeyIsNotRename:
     def test_title_slugs_keep_path_matching(self, slug):
         """Hub-first rows never claimed an identifier; the path is their only link."""
         assert not fp._slug_is_identifier(slug)
+
+
+class TestRetirementReleasesTheIdentifier:
+    """`slug` is unique table-wide, so retirement must free it.
+
+    Retirement that only sets a timestamp leaves the identifier locked to a
+    record nothing derives, and the repository that owns it can never claim it.
+    """
+
+    def test_the_identifier_is_released(self):
+        from datetime import datetime, timezone
+        when = datetime(2026, 8, 28, tzinfo=timezone.utc)
+        assert fp._tombstone_slug("repo-wp-0001", when) == "repo-wp-0001@retired-20260828"
+
+    def test_re_retiring_does_not_stack_marks(self):
+        """Otherwise the 100-char column overflows after a few passes."""
+        from datetime import datetime, timezone
+        a = fp._tombstone_slug("repo-wp-0001", datetime(2026, 8, 28, tzinfo=timezone.utc))
+        b = fp._tombstone_slug(a, datetime(2026, 9, 1, tzinfo=timezone.utc))
+        assert b == "repo-wp-0001@retired-20260901"
+        assert b.count(fp.RETIRED_SLUG_MARK) == 1
+
+    def test_a_long_slug_stays_within_the_column(self):
+        from datetime import datetime, timezone
+        out = fp._tombstone_slug("x" * 140, datetime(2026, 8, 28, tzinfo=timezone.utc))
+        assert len(out) <= 100