Story 18: map SRNE off-grid inverter lifecycle

[tushabe]

Summary

• add an SRNE OffGridBatteryInverter component
• read machine state 0x0210 and active power over FC03
• map stable machine states to OpenEMS grid mode and expose lifecycle state
• keep control and FC16 writes explicitly deferred to Story 19
• document that only state 2 is live-validated so far

Verification

• ./gradlew :io.openems.edge.ess.srne:test
• ./gradlew :io.openems.edge.ess.srne:check buildEdge buildAntoraDocs

Tracks Nearly-Free-Energy/nfe-modbus-energy-logger#18

Stacked on PR #3; rebase the base to develop after PR #3 merges.

[arindahills]

The OffGridBatteryInverter + StateMachine (machine-state → GridMode, only state 2 validated, transition/unknown states don't claim a mode) is exactly right and respects the live-validation discipline. Read-only + targetGridMode plumbing for story 19 is the right call. One change, same as #2:

ACTIVE_POWER here is also mapped from 0x021B (load power) — same issue: it's the load, not the inverter's power, and phase-A only. Derive it from battery DC power (0x0101 × 0x0102). There is no inverter/ESS power register on this model (verified deep-dive: https://bookstack.nearlyfreeenergy.com/books/product/page/srne-asp48120sh3-modbus-register-map-validated).