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/mcp/tools/projects.py

@@ -61,11 +61,14 @@ async def enter_project(project_id: int) -> dict:
     open_tasks, recent_notes, design_system, systems, pattern_coverage —
     plus systems_bootstrap, present only when it applies (see below).
 
-    `pattern_coverage` (usually null) is a one-line estimate of how much of
-    the bound repo's code has recorded snippets — e.g. "pattern-library
-    coverage: 34/210 shapes recorded (estimate); largest gaps: internal/api".
-    When present, treat the gaps as a standing invitation: as you touch code
-    in those areas, record the shapes you find with create_snippet.
+    `pattern_coverage` (usually null) is the shape-accounting line — how many
+    of the bound repo's extracted shapes carry a classification against canon
+    (note 2786) — e.g. "shape accounting: 3100/4573 shapes accounted for —
+    12 canonical · 2900 instance (estimate, computed 2026-08-19); 1473
+    unclassified, largest: internal/api". Unclassified IS the todo: as you
+    touch code in those areas, classify the shapes you can (instances of
+    recorded canon, deliberate variants, one-off exemptions) and record the
+    canon that's missing with create_snippet.
 
     `systems` is the project's vocabulary of named subsystems/areas. It is
     returned here so you can TAG as you write: when creating or meaningfully