feat(ledger): uses edges — consumption is its own relation, conformance keeps one snippet_id (#2870, milestone 294)

A shape can follow one convention canon AND call several helper canons; the
row's single snippet_id made the 2026-08 audit pick (hash_token won, the
service-function convention lost), and hook evidence — pulled a snippet, then
wrote code naming it — was stamped as instance when it is a uses fact.

- code_shape_uses (migration 0084): shape → snippet, basis, evidence; unique
  per pair; cascades with both ends. USE_BASES: reference | hook | agent |
  audit | import. A judgment-grade basis overwrites a mechanical one, never
  the reverse.
- classify_shapes items and classify_shapes_by_rule take uses=[snippet ids]
  (targets validated like snippet_id; all-or-nothing).
- The write-path hook writes a uses edge for every pulled canon the payload
  names (the instance stamp is unchanged); the proposer writes a uses edge
  for every canon a body names (reference_canons: kind + language family +
  stoplist, same rules as the reference basis) — the mechanical form of
  "auto-confirm own-import references" deferred from #2871.
- list_shapes(uses=N) lists the consumers of a canon; get_snippet's consumer
  map gains `uses` beside instances/variants.

Operator decision on #2870 (2026-08-21): keep one snippet_id, add uses edges.

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

alembic/versions/0084_code_shape_uses.py

@@ -0,0 +1,40 @@
+"""code_shape_uses — consumption edges, separate from conformance (#2870, milestone 294)
+
+Revision ID: 0084
+Revises: 0083
+Create Date: 2026-08-21
+
+A ledger row carries ONE snippet_id: what shape this is (instance/variant of
+a canon). But a shape can also CALL several canonical helpers — a service
+function that is an instance of the service-function convention and a
+consumer of hash_token. The 2026-08 audit had to pick one; hook evidence
+("pulled #N then wrote code referencing it") was stamped as instance when it
+is a uses fact. This table holds the many-valued relation: shape → snippet,
+with the basis and the evidence. Cascades with the shape and the snippet.
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0084"
+down_revision = "0083"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "code_shape_uses",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("shape_id", sa.Integer(), sa.ForeignKey("code_shapes.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("snippet_id", sa.Integer(), sa.ForeignKey("notes.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("basis", sa.Text(), nullable=False),
+        sa.Column("evidence", sa.Text(), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.text("now()")),
+        sa.UniqueConstraint("shape_id", "snippet_id", name="uq_code_shape_uses_shape_snippet"),
+    )
+    op.create_index("ix_code_shape_uses_snippet", "code_shape_uses", ["snippet_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_code_shape_uses_snippet", table_name="code_shape_uses")
+    op.drop_table("code_shape_uses")