feat(coverage): the line names standing work with 0 unclassified + derive_new, copies that joined a family since the previous refresh (#2899, milestone 299 step 1)

Since the scoped bucket (#2869) the ledger reads 100% accounted while 439
derive rows stand; the standing block was gated on unclassified > 0 and so
went silent. Build it whatever the todo count ("; standing: ..."), and add
derive_new — derive-grouped rows first seen after the previous refresh
stamp — so entering a project names the drift ("+2 new copies since last
refresh: .error-msg in InceptionCard.vue") instead of waiting for an audit.

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

tests/test_shape_ledger.py

@@ -317,6 +317,38 @@ def test_derive_groups_copy_before_name_with_floors():
     assert ("i.py", "sym", "one") not in g
 
 
+def test_derive_new_summary_counts_copies_first_seen_since_the_previous_refresh():
+    """#2899: the arrival-moment drift signal — derive-grouped rows created
+    after the previous refresh's stamp, newest first, judged rows and a
+    first seed (since=None) never count."""
+    from datetime import datetime, timedelta, timezone
+
+    from scribe.models.code_shape import CodeShape
+    from scribe.services.shape_ledger import derive_new_summary
+
+    t0 = datetime(2026, 8, 22, 12, 0, tzinfo=timezone.utc)
+
+    def row(path, symbol, at, kind="css", status="scoped", group="dup:abc", basis="derive"):
+        r = CodeShape(project_id=2, repo_key="r", path=path, symbol=symbol, kind=kind,
+                      status=status, proposal_basis=basis, proposal_group=group)
+        r.created_at = at
+        return r
+    rows = [
+        row("v/Old.vue", "error-msg", t0 - timedelta(days=3)),              # before the stamp
+        row("v/InceptionCard.vue", "error-msg", t0 + timedelta(hours=1)),   # new copy
+        row("v/Other.vue", "error-msg", t0 + timedelta(hours=2)),           # newer copy
+        row("v/J.vue", "error-msg", t0 + timedelta(hours=3), status="exempt"),   # judged: never
+        row("s/a.py", "load", t0 + timedelta(hours=1), kind="sym", group=None, basis=None),  # no family
+    ]
+    out = derive_new_summary(rows, since=t0)
+    assert out["count"] == 2
+    assert [e["path"] for e in out["examples"]] == ["v/Other.vue", "v/InceptionCard.vue"]
+    assert out["examples"][0] == {"label": ".error-msg", "path": "v/Other.vue", "group": "dup:abc"}
+    assert derive_new_summary(rows, since=None) == {"count": 0, "examples": []}
+    assert derive_new_summary(rows, since=t0, top=1)["examples"] == [
+        {"label": ".error-msg", "path": "v/Other.vue", "group": "dup:abc"}]
+
+
 def test_proposal_summary_ranks_body_identical_groups_first_and_sees_scoped_rows():
     """#2872: dup groups (the real copies) outrank name groups (usually
     convention), wider spread first; #2869: scoped rows are in the readout."""