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>

tests/test_integration_shape_classify.py

@@ -201,6 +201,41 @@ async def test_sync_stamps_scoped_rows_and_unstamps_when_they_become_reachable(s
     assert by_symbol["card"].status == "instance" and by_symbol["card"].snippet_id == sid
 
 
+@pytest.mark.integration
+async def test_uses_edges_are_the_consumer_map(seeded):
+    """#2870: a shape keeps ONE snippet_id (what it is) and any number of
+    uses edges (what it calls); the snippet's consumer map lists them,
+    list_shapes(uses=N) finds them, and a sweep can write them."""
+    from scribe.services import snippets as snippets_svc
+    owner, pid, sid = seeded["owner"], seeded["pid"], seeded["snippet"]
+    helper = await snippets_svc.create_snippet(
+        owner, name="cls_hash_helper", code="def hash_token(raw):\n    return raw\n",
+        language="python", repo="Widget", path="src/hash.py", symbol="hash_token",
+        project_id=pid,
+    )
+    hid = int(helper.id)
+    out = await classify_shapes(owner, pid, [
+        {"path": "src/app.py", "symbol": "make_app", "status": "instance",
+         "snippet_id": sid, "uses": [hid]},
+    ], via="audit")
+    assert out["classified"] == 1
+    rows, total = await list_project_shapes(owner, pid, uses=hid)
+    assert total == 1 and rows[0].symbol == "make_app" and rows[0].snippet_id == sid
+    consumers = await snippet_consumers(owner, hid)
+    assert consumers["instances"] == [] and len(consumers["uses"]) == 1
+    assert consumers["uses"][0]["symbol"] == "make_app" and consumers["uses"][0]["basis"] == "audit"
+    # A sweep writes uses too; an unknown snippet in uses applies nothing.
+    out = await classify_shapes_where(
+        owner, pid, path="src/util.py", status="exempt", reason="local", uses=[hid],
+    )
+    assert out["classified"] == 1
+    assert (await list_project_shapes(owner, pid, uses=hid))[1] == 2
+    with pytest.raises(ValueError):
+        await classify_shapes(owner, pid, [
+            {"path": "src/app.py", "symbol": "Config", "status": "exempt", "reason": "x", "uses": [999999]},
+        ])
+
+
 @pytest.mark.integration
 async def test_list_filters_compose(seeded):
     owner, pid, sid = seeded["owner"], seeded["pid"], seeded["snippet"]