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>

docs/api-reference.md

@@ -173,7 +173,7 @@ endpoint at `/mcp`, not these REST routes.
 | GET | `/api/plugin/retrieve` | Title-first knowledge-injection candidates |
 | GET | `/api/plugin/processes` | Stored Processes for skill-stub sync |
 | GET | `/api/plugin/prior-art` | Write-path hint for the plugin hooks (params: `path`, `code`, `repo`, `shapes`, `exclude_ids`, `exclude_sync_ids`, `exclude_derive`); returns `context`, `note_ids`, `sync_note_ids`, `stamped`, `divergence`, `derive`, `derive_keys` |
-| GET / POST | `/api/projects/<id>/coverage`, `…/coverage/refresh` | Shape-ledger accounting (`pattern_coverage` line, counts, `derive_groups`, `derive_new`, `divergence`, `recheck`) |
+| GET / POST | `/api/projects/<id>/coverage`, `…/coverage/refresh` | Shape-ledger accounting (`pattern_coverage` line, counts, `derive_groups` — css groups carry `consumers`, `derive_new`, `unused_css`, `divergence`, `recheck`) |
 | GET / PUT | `/api/plugin/marketplace-url` | Read / set the plugin marketplace URL |
 
 ## Dashboard, Export, Trash, Users