feat(ledger): the consumer map surfaces — every css row carries used_by on list_shapes, flag="unused-css" (the map's negative space, surfaced never deleted), derive groups and the write-path family carry consumers, the derive line says "used by N template(s)", coverage payload unused_css + the standing block; docs, SKILL, plugin 0.1.44; the two step-2 tests expected 5 edges where fan-out makes 6 (milestone 302 step 3, #2936)

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

src/scribe/services/coverage.py

@@ -629,7 +629,23 @@ async def compute_coverage(
         agg["accounted"] += row.status != "unclassified"
 
     unclassified = counts.pop("unclassified")
-    proposals = shape_ledger.proposal_summary(rows)
+    # The CSS consumer map's readout (milestone 302): which files render each
+    # css row — on the derive groups (a shared recipe vs a scoped one is a
+    # count), and the negative space: css rules no template names. "Unused"
+    # is measured only where the map has evidence of templates at all (one
+    # edge somewhere); a repo of bare stylesheets is "not measured", not
+    # "all unused".
+    css_rows = [r for r in rows if r.kind == "css"]
+    consumer_paths: dict[int, list[str]] = {}
+    unused_css = None
+    try:
+        edges = await shape_ledger.consumers_of([r.id for r in css_rows])
+        consumer_paths = {sid: [e.path for e in es] for sid, es in edges.items()}
+        if consumer_paths:
+            unused_css = sum(1 for r in css_rows if r.id not in consumer_paths)
+    except Exception:
+        logger.warning("consumer map read failed", exc_info=True)
+    proposals = shape_ledger.proposal_summary(rows, consumer_paths=consumer_paths)
     divergence = shape_ledger.divergence_summary(rows)
     derive_new = shape_ledger.derive_new_summary(rows, since=since)
     return {
@@ -646,6 +662,9 @@ async def compute_coverage(
         # duplicate family — what the arrival line names so drift is noticed
         # on entering, not found by an audit.
         "derive_new": derive_new,
+        # The consumer map's negative space (milestone 302): live css rules
+        # no template names — None when the map has no evidence of templates.
+        "unused_css": unused_css,
         "proposer": proposer_stats,
         # The divergence readout (#2793): button B where button A is canon,
         # and judged shapes whose bodies moved since they were judged.
@@ -823,7 +842,16 @@ def coverage_line(coverage: dict) -> str:
         standing.append(f"top canon #{top['snippet_id']} ×{top.get('count', 0)}")
     first = (coverage.get("derive_groups") or [{}])[0]
     if first.get("label") and first.get("files"):
-        standing.append(f"top copy {first['label']} ×{first['files']} files")
+        top_copy = f"top copy {first['label']} ×{first['files']} files"
+        # A css family says what renders it (milestone 302): the count that
+        # tells a shared recipe from a scoped convention.
+        if "consumers" in first:
+            n_t = (first.get("consumers") or {}).get("count", 0)
+            top_copy += f" · used by {n_t} template{'s' if n_t != 1 else ''}"
+        standing.append(top_copy)
+    if coverage.get("unused_css"):
+        n_u = coverage["unused_css"]
+        standing.append(f"{n_u} unused class{'es' if n_u != 1 else ''}")
     if unclassified:
         line += f"; {unclassified} unclassified"
         if standing: