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