fix(ledger): divergence asks at family level — the first gate was inverted (#4204)

CI run 7090 caught this; the unit suite could not. Integration job 25763 —
`test_a_second_confirm_dialog_is_detected_and_named`, the acceptance case of
milestone #2793.

WHAT I GOT WRONG. The previous commit gated BOTH halves of the ledger on
`forms_agree`. That is right for stamping and backwards for divergence,
because the two assert opposite things:

  STAMPING says "this IS that canon". Agreement in form is evidence FOR the
  claim, so demanding it is correct.

  DIVERGENCE says "this is NOT the canon that dominates here — did you mean
  to?" A form MISMATCH is the PREMISE of that prompt. Requiring the candidate
  to match the canon silences the check precisely where it belongs.

So #2793's case stopped firing: a hand-rolled sync `confirmDanger` in a
directory where an async confirm helper is canon read as `fn` against
`async-fn`, disagreed, and was dropped. `flag_divergence` returned 0 where the
test demands 1, and the write-time check returned nothing where it must name
the canon. That is a real flag the milestone exists to produce, and my change
removed it.

THE FIX. Divergence now gates at FAMILY level — callable {fn, async-fn},
type, value, css — and only on contradiction. A sync function beside an async
one is still a fair question. A frozen dataclass told to build from an async
service function is not a question at all.

WHAT THIS DOES NOT FIX, asserted rather than commented so it fails the day it
changes (`test_how_many_of_the_five_the_divergence_gate_actually_silences`):
of #4204's five false prompts this silences ONE. `Point` is a type against a
callable canon. `_p`, `get_point`, `is_registered` and
`sources_expected_to_emit` are callables like the canon and still ask — and
at the signature level they are indistinguishable from the #2793 case above,
so nothing readable here can separate them. That needs #4204 option 2 (widen
`kind` past `css | sym`) or a comparison of meaning rather than form.

The stamping half — `_RESEMBLE_MIN` 0.80 and the graded burden — is unchanged
and unaffected by this failure. It is also the half that matters more: loose
stamping is what manufactures the density the divergence check reads as
authority.

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

src/scribe/services/shape_ledger.py

@@ -987,6 +987,59 @@ def forms_conflict(a: str, b: str) -> bool:
     return bool(a) and bool(b) and a != b
 
 
+def shape_family(form: str) -> str:
+    """The coarse category a form belongs to, or "" when the form is unknown.
+
+    WHY THIS IS SEPARATE FROM `shape_form`, and it is the correction to the
+    first version of this change. The two checks in this module ask opposite
+    questions, so they cannot share a burden:
+
+      STAMPING asserts "this IS that canon". Agreement in form is evidence
+      FOR the claim, so `forms_agree` / `forms_conflict` — the precise level,
+      where `fn` and `async-fn` are different — is right.
+
+      DIVERGENCE asserts "this is NOT the canon that dominates here — did you
+      mean to?" A form MISMATCH is the premise of that prompt, not an
+      objection to it. Gating it on `forms_agree` inverted the check: it went
+      silent on exactly the mismatches it exists to catch, and the acceptance
+      case of #2793 — a hand-rolled sync `confirmDanger` in a directory where
+      an async confirm helper is canon — stopped being flagged.
+
+    So divergence gates at FAMILY level and only on contradiction. A sync
+    function beside an async one is still a fair question. A frozen dataclass
+    told to build from an async service function is not a question at all,
+    and that is the #4204 prompt this removes.
+
+    This does NOT remove every false prompt #4204 recorded. Of the five, it
+    silences `Point` (a type, against a callable canon); `_p`, `get_point`,
+    `is_registered` and `sources_expected_to_emit` are callables like the
+    canon and still ask. At the signature level they are indistinguishable
+    from the #2793 case above, so nothing readable here can separate them —
+    only widening `kind` past `css | sym` (#4204 option 2) or comparing
+    meaning rather than form can. Stated here so the next reader does not
+    assume the gap is an oversight.
+    """
+    if form in ("fn", "async-fn"):
+        return "callable"
+    if form == "type":
+        return "type"
+    if form == "binding":
+        return "value"
+    if form == "css":
+        return "css"
+    return ""
+
+
+def families_conflict(a: str, b: str) -> bool:
+    """Do two forms belong to categorically different KNOWN families?
+
+    Like `forms_conflict`, both sides must be readable: an unknown form makes
+    this quieter, never more confident.
+    """
+    fa, fb = shape_family(a), shape_family(b)
+    return bool(fa) and bool(fb) and fa != fb
+
+
 def canon_form(rows: Iterable, snippet_id: int) -> str:
     """The form a canon's own judged rows agree on, or FORM_UNKNOWN."""
     forms: dict[str, int] = {}
@@ -1996,18 +2049,22 @@ async def write_time_divergence(
             row.status != "unclassified" or row.proposed_snippet_id == sid
         ):
             continue
-        # DOES THIS SHAPE EVEN RESEMBLE THE CANON? (#4204) Without this the
-        # line was a pure base rate: "most things here are X, so be X", with
-        # the candidate never examined. It told a frozen dataclass and three
-        # pure predicates to build from the `async_session` service canon.
+        # IS THIS EVEN THE SAME CATEGORY OF THING AS THE CANON? (#4204)
+        # Without this the line was a pure base rate: "most things here are X,
+        # so be X", with the candidate never examined at all. It told a frozen
+        # dataclass to build from the `async_session` service canon.
+        #
+        # FAMILY, not form, and only on contradiction — see `shape_family`. A
+        # divergence prompt is ABOUT a mismatch, so requiring the candidate to
+        # match would silence the check precisely where it belongs.
         #
         # Signature from the payload first — a shape being written now may
-        # have no row yet, and `forms_agree` needs both sides known, so an
-        # unreadable one produces silence rather than a guess.
+        # have no row yet — and an unreadable one produces a fair question
+        # rather than a guess, because `families_conflict` needs both sides.
         mine = shape_form(
             signature_in(code, name, kind) or getattr(row, "signature", "") or "", kind
         )
-        if not forms_agree(mine, cform):
+        if families_conflict(mine, cform):
             continue
         out.append({"symbol": name, "kind": kind, "canon_snippet_id": sid,
                     "instances": n, "judged": judged})
@@ -2129,7 +2186,7 @@ async def flag_divergence(project_id: int, *, since: datetime | None) -> int:
                 # (#4204). The sweep and the hook must agree about what counts
                 # as divergence, or an audit contradicts the line the writer
                 # was shown at the keyboard.
-                if not forms_agree(shape_form(r.signature or "", r.kind), cform):
+                if families_conflict(shape_form(r.signature or "", r.kind), cform):
                     continue
                 r.diverges_from = dom[0]
                 flagged += 1

tests/test_shape_form_gate.py

@@ -26,7 +26,8 @@ from __future__ import annotations
 import pytest
 
 from scribe.services.shape_ledger import (
-    FORM_UNKNOWN, canon_form, forms_agree, shape_form, signature_in,
+    FORM_UNKNOWN, canon_form, families_conflict, forms_agree, shape_family,
+    shape_form, signature_in,
 )
 
 
@@ -89,6 +90,65 @@ def test_the_five_shapes_that_started_this_are_not_service_functions() -> None:
         assert not forms_agree(shape_form(sig, "sym"), canon), sig
 
 
+def test_how_many_of_the_five_the_divergence_gate_actually_silences() -> None:
+    """HONEST ACCOUNTING, and it is deliberately an assertion rather than a
+    comment so it fails the day the answer changes.
+
+    Divergence gates at FAMILY level and only on contradiction, because a form
+    mismatch is the PREMISE of a divergence prompt, not an objection to it —
+    see `shape_family`. So of #4204's five false prompts this silences one:
+    `Point` is a type and the canon is a callable. The four `def` helpers are
+    callables like the canon and still ask.
+
+    That is not a shortcoming of the gate, it is the limit of the signature:
+    at this level those four are indistinguishable from #2793's acceptance
+    case, where a sync `confirmDanger` beside an async confirm helper SHOULD
+    be flagged. Separating them needs #4204 option 2 (widen `kind`) or a
+    comparison of meaning."""
+    canon = shape_form("async def create_note(user_id: int, ...):", "sym")
+    silenced = {
+        sig: families_conflict(shape_form(sig, "sym"), canon)
+        for sig in (
+            "class Point:",
+            "def _p(source, kind, what, **kw) -> tuple[str, Point]:",
+            "def get_point(source: str) -> Point | None:",
+            "def is_registered(source: str) -> bool:",
+            "def sources_expected_to_emit() -> list[str]:",
+        )
+    }
+    assert silenced["class Point:"] is True
+    assert sum(silenced.values()) == 1
+
+
+def test_a_sync_function_beside_an_async_canon_is_still_a_fair_question() -> None:
+    """#2793's acceptance case, which the first version of this change broke:
+    gating divergence on `forms_agree` silenced a hand-rolled sync confirm in
+    a directory where an async confirm helper is canon — the exact prompt the
+    milestone exists to produce."""
+    canon = shape_form("async function onTrash() {", "sym")
+    mine = shape_form("function confirmDanger() {", "sym")
+    assert canon == "async-fn" and mine == "fn"
+    assert not forms_agree(mine, canon)      # the STAMP gate would refuse
+    assert not families_conflict(mine, canon)  # the DIVERGENCE gate asks anyway
+
+
+@pytest.mark.parametrize(("form", "want"), [
+    ("fn", "callable"), ("async-fn", "callable"), ("type", "type"),
+    ("binding", "value"), ("css", "css"), (FORM_UNKNOWN, ""),
+])
+def test_every_form_has_a_family_and_unknown_has_none(form, want) -> None:
+    assert shape_family(form) == want
+
+
+@pytest.mark.parametrize(("a", "b"), [
+    (FORM_UNKNOWN, "fn"), ("fn", FORM_UNKNOWN), (FORM_UNKNOWN, FORM_UNKNOWN),
+])
+def test_an_unreadable_form_never_silences_a_divergence_prompt(a, b) -> None:
+    """Same direction as everywhere else in this module: not knowing makes the
+    check quieter about ASSERTING and never quieter about ASKING."""
+    assert not families_conflict(a, b)
+
+
 # ── forms_agree: unknown never matches ────────────────────────────────────
 
 def test_two_known_equal_forms_agree() -> None: