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

src/scribe/services/shape_ledger.py

@@ -1570,12 +1570,22 @@ def _semantic_priority(row) -> tuple:
 # not evidence (see _SEMANTIC_LIMIT) — but the proposals themselves did not
 # change, so no re-examination is owed; a stale miss basis is inert.
 # v5: the semantic arm's floor dropped from 0.8 to the write-path floor (#4208),
-# so rows it examined and found nothing for must be read once more.
+# and the signature floor from 0.8 to 0.75 (#4306), so rows examined and
+# found nothing for must be read once more.
 _PROPOSER_VERSION = 5
 # Signature resemblance floor, name blanked (difflib ratio) — and a length
 # floor, because `def NAME():` resembles `def NAME(x):` at 0.95 while saying
 # nothing; a family shape has parameters to resemble.
-_SIGNATURE_FLOOR = 0.8
+#
+# A proposal is read before it is confirmed, so this admits the band where
+# true instances live rather than the band where nothing is wrong (#4306).
+# Measured 2026-09-22 over 459 judged instance rows against every same-family
+# canon: true instances score 1.0 or ~0.795 (a model class whose mixin list
+# differs from the canon's), and 0.8 cut the whole second band. At 0.75 all
+# of them clear; the price is `async def NAME(user_id: int, …) -> dict`
+# look-alikes (35 wrong-canon pairings vs 14 at 0.8), which read as wrong at
+# a glance. Below 0.75 the next true row is at 0.6 behind 150+ wrong ones.
+_SIGNATURE_FLOOR = 0.75
 _SIGNATURE_MIN_LEN = 30
 # Textual containment needs enough substance to mean anything.
 _TEXT_FLOOR = 40

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