feat(ledger): coverage refresh feeds the shape ledger; the readout inverts to accounting (#2788, milestone 294 step 2)

compute_coverage is now the ledger's sync point: every walk upserts the
extracted shapes (new → unclassified, the todo state; surviving → last-seen
bump; vanished → stamped, kept as history), re-files judgments whose snippet
target went away, and mechanically stamps snippet reference locations as
canonical — the one always-safe rule, self-healing only for its own stamps
(an agent's judgment is never unwound by machinery).

The covering predicate moves to shape_ledger.location_covers as the single
home (match_shapes retired with its consumer); coverage's payload and line
invert from 'N/M shapes recorded' to shape ACCOUNTING per note 2786:
accounted/total with a canonical·instance·variant·exempt breakdown, and
unclassified — THE todo — with its largest directories. Cache key bumps to
v2 so pre-ledger blobs honestly read 'not measured yet' instead of rendering
in a shape no longer spoken.

Readout is deliberately project-wide (all repos' live rows), while the walk
serves whichever repos the owner's keyring reaches this refresh.

Integration tests pin the new contract: rows for every extracted shape,
mechanical canonical stamps carrying snippet ids, idempotent recompute,
agent judgments surviving recompute AND vanish/return, vanished rows leaving
the readout but keeping their history.

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

src/scribe/routes/projects.py

@@ -123,7 +123,7 @@ async def delete_project_route(project_id: int):
 @projects_bp.route("/<int:project_id>/coverage", methods=["GET"])
 @login_required
 async def get_coverage_route(project_id: int):
-    """The cached pattern-library coverage summary — never computes.
+    """The cached shape-accounting summary — never computes.
 
     `configured` tells the card whether offering a Refresh button makes
     sense; `coverage` is null until something has computed it (a webhook