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>

src/scribe/services/shape_ledger.py

@@ -1404,6 +1404,34 @@ def proposal_summary(rows: Iterable[CodeShape], *, top: int = 8) -> dict:
     return {"proposed": proposed, "derive_groups": ranked[:top], "top_canon": top_canon}
 
 
+def derive_new_summary(
+    rows: Iterable[CodeShape], *, since: datetime | None, top: int = 3
+) -> dict:
+    """The arrival-moment drift signal (#2899): derive-grouped rows FIRST
+    SEEN after ``since`` — the previous refresh's stamp, the same one
+    flag_divergence uses. "Since the last refresh, N more copies joined a
+    duplicate family" is the sentence that makes the derive queue a thing
+    you notice on entering, not a thing an audit finds. ``since`` None (a
+    first seed) means nothing is new. Judged rows never count."""
+    if since is None:
+        return {"count": 0, "examples": []}
+    fresh = [
+        r for r in rows
+        if r.proposal_basis == "derive" and r.proposal_group
+        and r.status in _MECHANICAL_TODO and r.vanished_at is None
+        and r.created_at is not None and r.created_at > since
+    ]
+    fresh.sort(key=lambda r: r.created_at, reverse=True)
+    return {
+        "count": len(fresh),
+        "examples": [
+            {"label": ("." if r.kind == "css" else "") + r.symbol,
+             "path": r.path, "group": r.proposal_group}
+            for r in fresh[:top]
+        ],
+    }
+
+
 async def confirm_proposals(
     user_id: int,
     project_id: int,