ARCH-02: assert the invariant, because a fixture cannot enumerate arrangements

Review approved and then attacked part 5b, the control that stands between B1's
defect and a green suite. It got through in three edits:

    A1  delete `END LEG`                        5b CATCHES it        2 FAIL
    A2  ...and reorder the legs                 5b BLIND             1 FAIL
    A3  ...and reword the closing summary       0 FAIL, "all checks passed"
                                                with a splitter that closed
                                                ZERO legs

⚠️ TWO OF THOSE THREE ARE ORDINARY REFACTORS - reordering report legs, rewording
a summary line - and neither reads as touching a safety check. 5b caught the
unclosed splitter only because the clean leg happened to be printed LAST, and
the backstop that saved A2 was a hardcoded transcription of the tool's own
sentence, so rewording that sentence blinded it too.

FIXED BY ASSERTING THE STRUCTURE RATHER THAN ADDING A FIXTURE: a leg may not
open while one is open, and every leg that opens must be explicitly closed.
Driven - deleting `END LEG` now fails on the FIRST edit, whatever the order or
the wording:

    AssertionError: leg 'SAME-STEP' opened while 'STAGED' was still open - the
    tool stopped closing its legs, and a splitter that never closes swallows the
    closing summary into the last leg.

All three attacks include that edit, so one assertion kills the class. Clean
tree unchanged: 28/28, all checks passed.

⭐ AND THE RULE IN MUTATION.md IS EXTENDED, because review's finding is the
general case of it. It said "a per-thing check that has never seen the things
DISAGREE is a total". Part 5b DOES make them disagree, every run, and was still
blind. So:

    ...AND ONE THAT HAS ONLY SEEN THEM DISAGREE IN ONE ARRANGEMENT IS A TOTAL
    OVER THAT ARRANGEMENT. When a fixture cannot enumerate the arrangements,
    assert the invariant they share.

Also recorded, NOT fixed, with the reason: the reference carries `entity_id
unique_id device` and grades all three, but never the discovery TOPIC. Review
measured `homeassistant/... -> ha/...` exiting 0 with no findings while every
announced entity dies. `discovery_payloads()` already returns the topic and
`announced_rows` discards it, so a fourth column closes it - but the prefix is
HA's fixed contract rather than a name this project chooses, so it is not what a
tidy-up rename reaches. Lower LIKELIHOOD than the N1 gap it resembles, not lower
consequence, and it stops being a note the day the topic becomes configurable.

Two stale figures corrected: the tool claimed five migration mutations where
there are nine, and pointed at a DOCS.md heading that does not exist (the real
one is "Before you commission inverter B", verified present at DOCS.md:1577).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwRLw7eKYj2byVdwB6kbjU

goodwe_controller/MUTATION.md

@@ -283,6 +283,20 @@ same place twice, the check is decoration.
     has never seen the things DISAGREE is a total.** Part 5b exists to make
     them disagree on every run.
 
+    ⚠️ **AND ONE THAT HAS ONLY SEEN THEM DISAGREE IN ONE ARRANGEMENT IS A TOTAL
+    OVER THAT ARRANGEMENT.** Review found this by attacking the fix itself. Part
+    5b does make the legs disagree, on every run - and was still blind to a build
+    that merely REORDERS them, because it had only ever seen the clean leg
+    printed LAST. Three edits, two of them ordinary refactors (reorder the report
+    legs, reword a summary line), produced `all checks passed` from a splitter
+    that closed ZERO legs.
+
+    A fixture cannot enumerate the arrangements, so the STRUCTURE is asserted
+    instead: a leg may not open while one is open, and every leg that opens must
+    be explicitly closed. That fires on the FIRST edit of all three attacks,
+    whatever the order or the wording. **When a fixture cannot enumerate the
+    arrangements, assert the invariant they share.**
+
 * **CFG-04's `tools/supervisor_shape_probe.py` §B** — and this one is the rule
   reaching outside the suite. The probe is the committed evidence that
   `config.yaml`'s `inverters: []` default is load-bearing, and four documents