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>

src/scribe/models/__init__.py

@@ -44,6 +44,6 @@ from scribe.models.rulebook import (  # noqa: E402, F401
 )
 from scribe.models.repo_binding import RepoBinding  # noqa: E402, F401
 from scribe.models.forge_connection import ForgeConnection  # noqa: E402, F401
-from scribe.models.code_shape import CodeShape, CodeShapeEvent  # noqa: E402, F401
+from scribe.models.code_shape import CodeShape, CodeShapeEvent, CodeShapeUse  # noqa: E402, F401
 from scribe.models.system import System, RecordSystem  # noqa: E402, F401
 from scribe.models.design_system import DesignSystem, DesignToken  # noqa: E402, F401

src/scribe/models/code_shape.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 
 from sqlalchemy import (
     BigInteger,
@@ -217,6 +217,54 @@ class CodeShape(Base, TimestampMixin):
         return out
 
 
+# How a uses edge was established (#2870): who/what said "this shape calls
+# that canon". `reference` is the proposer's mechanical by-name hit on the
+# body (language-gated, #2871); `hook` is write-path evidence (pulled the
+# snippet, then wrote code naming its symbol); agent/audit/import are
+# judgments carried on classify_shapes(..., uses=[...]).
+USE_BASES = ("reference", "hook", "agent", "audit", "import")
+
+
+class CodeShapeUse(Base):
+    """One consumption edge: shape → canonical snippet it calls/uses (#2870).
+
+    Conformance (CodeShape.status/snippet_id) answers "what shape is this";
+    this table answers "what does it use" — many per shape. A service function
+    that is an instance of the service-function convention AND a consumer of
+    hash_token has one snippet_id and one uses edge. Cascades with both ends:
+    a use of a deleted snippet is no longer a fact worth keeping.
+    """
+
+    __tablename__ = "code_shape_uses"
+    __table_args__ = (
+        UniqueConstraint("shape_id", "snippet_id", name="uq_code_shape_uses_shape_snippet"),
+        Index("ix_code_shape_uses_snippet", "snippet_id"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    shape_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("code_shapes.id", ondelete="CASCADE"), nullable=False
+    )
+    snippet_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("notes.id", ondelete="CASCADE"), nullable=False
+    )
+    basis: Mapped[str] = mapped_column(Text, nullable=False)
+    evidence: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, default=lambda: datetime.now(timezone.utc)
+    )
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "shape_id": self.shape_id,
+            "snippet_id": self.snippet_id,
+            "basis": self.basis,
+            "evidence": self.evidence,
+            "created_at": iso(self.created_at),
+        }
+
+
 # What a shape's history records (#2793). Not "appeared" — first_seen and
 # created_at already say that on the row; history is for what CHANGED:
 SHAPE_EVENTS = ("classified", "vanished", "reappeared", "drifted")