Precedence arbiter: one rule instead of an if/else ladder

Two controllers writing one actuator is the failure this system exists to
avoid. "Exactly one writer" was true, but only as a convention held up by
careful reading - which does not survive an EV charger and a heat pump wanting
the same battery.

Strategies now return claims and arbiter.py resolves them:

  highest-priority `set` wins (none at all means 0 W), then every `limit` whose
  priority is >= that set's applies, most restrictive first; contradictory
  limits command 0 W and are flagged as the bug they are.

The second clause is the whole point. "Money outranks maintenance" used to be a
hand-written exception inside a Jinja template; it is now a consequence of the
priorities - the charge-only limit binds the loop but cannot bind a
higher-priority peak claim.

Also: maintenance shaping moved out of control.py, which is a controller again
and not a policy engine; the loop now tracks the arbiter's actual output rather
than its own last wish, so it does not jump when it regains control; and every
decision explains itself ("loop -> 0 W, limited by maintenance(charge-only)")
in the UI and the log.

19 new assertions in test_arbiter.py, each one a precedence question someone
will eventually ask in the field. Deployed to the reference site as 0.2.0 and
holding grid within a few watts of zero.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016NckgXecasQb2eSsPYNSW6

goodwe_controller/config.yaml

@@ -1,5 +1,5 @@
 name: GoodWe RS485 Controller
-version: "0.1.6"
+version: "0.2.0"
 slug: goodwe_controller
 description: >-
   Drives a GoodWe ES/BP battery inverter over RS485 by emulating its smart