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>

tests/test_write_path_trigger.py

@@ -1306,7 +1306,8 @@ async def test_the_write_time_derive_check_names_a_family_or_a_canon_in_band():
         {"symbol": "log-empty", "kind": "css", "key": "name:css:log-empty",
          "family": {"group": "name:css:log-empty", "label": ".log-empty", "identical": False,
                     "files": ["a/TaskLogSection.vue", "a/WorkspaceTaskPanel.vue"],
-                    "file_count": 5, "size": 6}},
+                    "file_count": 5, "size": 6,
+                    "consumers": {"count": 6, "paths": ["a/TaskLogSection.vue", "a/V.vue"]}}},
         {"symbol": "btn-primary", "kind": "css", "key": "canon:2855",
          "canon": {"snippet_id": 2855, "path": "frontend/src/assets/components.css",
                    "label": ".btn-primary"}},
@@ -1344,8 +1345,10 @@ async def test_the_write_time_derive_check_names_a_family_or_a_canon_in_band():
     assert "Shape ledger at `frontend/src/components/New.vue`" in ctx
     # A CSS family is a repeated NAME (note 2917) and its dismissal is scoped-css;
     # a code dup family is an identical body and dismisses as convention-plumbing.
+    # A css family says what renders it (milestone 302) before the ask.
     assert "`.log-empty` is a repeated name with no canon — defined in 5 other file(s): " \
-           "`a/TaskLogSection.vue`, `a/WorkspaceTaskPanel.vue` +3 more; derive it now" in ctx
+           "`a/TaskLogSection.vue`, `a/WorkspaceTaskPanel.vue` +3 more; used by 6 templates: " \
+           "`a/TaskLogSection.vue`, `a/V.vue` +4 more; derive it now" in ctx
     assert "`slugify` is a duplicate family with no canon — identical body in 2 other file(s): " \
            "`a/x.py`, `a/y.py`; derive it now" in ctx
     assert "`.btn-primary` is canon — snippet #2855 at `frontend/src/assets/components.css`" in ctx