fix(shapes): the signature basis admits the band true instances score in (#4306)

Measured over 459 judged instance rows against every same-family canon:
true instances score 1.0 or ~0.795 (a model whose mixin list differs from
the canon's), and 0.8 cut the second band entirely. The proposal is read
before it is confirmed, so the floor admits it: 0.75 takes every measured
true row, at 35 wrong-canon pairings instead of 14.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_shape_ledger.py

@@ -180,6 +180,22 @@ def test_signature_similarity_blanks_the_names():
     assert sim("def helper(x):", "helper", "def make_app():", "make_app") == 0.0
 
 
+def test_a_model_with_a_different_mixin_list_is_proposed_against_the_model_canon():
+    """The band 0.8 cut (#4306). Measured over judged instances: a model class
+    whose mixins differ from the canon's scores ~0.795 against it — a true
+    instance, and the proposal is read before it is confirmed, so the floor
+    must admit it."""
+    from scribe.services.shape_ledger import Canon, _norm_text, match_canon
+
+    model = Canon(44, "sym", "Gadget", (("src/app/models/gadget.py", "Gadget"),),
+                  "class Gadget(Base, TimestampMixin, SoftDeleteMixin):",
+                  _norm_text(""), 0, "python")
+    hit = match_canon("sym", "src/app/models/widget.py", "Widget",
+                      "class Widget(Base, TimestampMixin):", "    pass", [model])
+    assert hit is not None and hit[:2] == (44, "signature")
+    assert 0.75 <= hit[2] < 0.8
+
+
 def test_text_containment_is_whitespace_insensitive_with_a_floor():
     from scribe.services.shape_ledger import text_contains