Release v2.3.1

Probe registers 30088/30200/30228/30286 (FC04, plant slave) on the
first poll cycle to detect the highest supported Modbus protocol level
(V2.6/V2.7/V2.8/V2.9). Also reads inverter firmware version string
from register 30525. Logs result as info-level message and persists it
to a new info.protocolLevel state — useful for GitHub issue triage.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com