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_integration_shape_classify.py

@@ -597,6 +597,39 @@ async def test_derive_groups_land_on_rows_and_in_the_summary(seeded):
     assert {r.symbol for r in rows} == {"slug"}                   # 2 files < the name floor
 
 
+@pytest.mark.integration
+async def test_derive_new_names_the_copy_that_joined_a_family_since_the_stamp(seeded):
+    """#2899: the first sync seeds one `slug`; a later sync adds an identical
+    copy. Against the stamp between them, derive_new counts ONLY the
+    newcomer — the drift since the last refresh, not the whole family."""
+    from datetime import datetime, timezone
+
+    from scribe.services.shape_ledger import (
+        apply_derive_groups, derive_new_summary, live_rows,
+    )
+
+    owner, pid = seeded["owner"], seeded["pid"]
+    first = _defs(
+        ("a/one.py", "sym", "slug", "def slug(t):", "def slug(t):\n    return t.lower()"),
+    )
+    await sync_repo_shapes(pid, REPO, first, seen_marker="m1")
+    stamp = datetime.now(timezone.utc)
+    second = _defs(
+        ("a/one.py", "sym", "slug", "def slug(t):", "def slug(t):\n    return t.lower()"),
+        ("a/two.py", "sym", "slug", "def slug(t):", "def slug(t):\n    return t.lower()"),
+    )
+    await sync_repo_shapes(pid, REPO, second, seen_marker="m2")
+    assert await apply_derive_groups(pid) == 2
+
+    rows = await live_rows(pid)
+    out = derive_new_summary(rows, since=stamp)
+    assert out["count"] == 1
+    assert out["examples"][0]["path"] == "a/two.py"
+    assert out["examples"][0]["label"] == "slug"
+    assert out["examples"][0]["group"].startswith("dup:")
+    assert derive_new_summary(rows, since=None)["count"] == 0
+
+
 # --- #2793: the divergence readout against real rows -------------------------