MAINT-04 review: pin the default divergence with a check, not a comment

validate.py's comment said "if config.yaml's value moves, move this and
main.py's together" and nothing enforced it - a comment where a check
belonged, in a PR that elsewhere proves it knows the difference.

test_validate.py now reads main.py and maintenance.py directly and asserts
the divergence EXACTLY AS IT STANDS: config.yaml/validate.py at 2000,
main.py's two fallbacks and MaintConfig's dataclass default at 2500. It
records the mismatch rather than demanding it be closed, so it passes today
and goes red if either side moves - including when somebody closes it in
main.py, at which point the check and validate.py's block are deleted
together. Nothing in a file another ticket owns is touched.

The consequence is asserted through the real code as well as the literals:
with the key absent the validator accepts, MaintConfig builds 2500 W against
a 2000 W ceiling, and the arbiter resolves it to 0 W as a contradiction -
the bug rule 6 exists to catch, passing rule 6. Production-unreachable (the
Supervisor always supplies every key) but fully reachable from sim/inject.py
and the tests, which is where anyone debugging maintenance works.

Also: a CHANGELOG section for the upgrade path, covering both the case where
the Supervisor re-derives the default and the case where it persisted the
old one, since which applies was not verifiable without probing the live
install. And maintenance.py's timeout comment said 13.5 kWh where everything
else says 13.8 (300 Ah x 51.2 V, DoD 90); SAFETY-18 does not touch that file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015VXjA3LWccN1rJsk3okyPC

goodwe_controller/app/validate.py

@@ -327,8 +327,12 @@ def validate_startup_config(opts: dict, dev_min=None, dev_max=None) -> list[str]
     # rule at 2000 and then built by main.py at 2500 - the contradiction this
     # rule exists to catch, in the one case it cannot see. It is unreachable on
     # a Supervisor install and those files are another ticket's right now.
-    # THE REAL FIX is one source of truth for option defaults; until then, if
-    # config.yaml's value moves, move this and main.py's together.
+    # THE REAL FIX is one source of truth for option defaults; until then the
+    # divergence is PINNED BY A CHECK rather than by this comment - see
+    # test_validate.py, "the KNOWN cross-file default divergence". It asserts
+    # the mismatch as it stands and goes red if EITHER side moves, including
+    # when somebody closes it in main.py, at which point this block and that
+    # check both get deleted.
     charge_w = _number(opts, "maintenance_charge_w", 2000.0)
     if charge_w is None:
         problems.append(