Fix Gateway auto-config binding PV-only inverters

[mgazza]

Summary

• defer Gateway auto-config when startup telemetry has no battery-capable inverter yet
• retry on the next telemetry frame instead of binding every discovered unit as a writable fallback
• add a two-frame regression test for a primary-flagged PV-only unit beside a battery inverter

Verification

• regression fails on unmodified main and passes with this change
• Gateway component suite passes
• quick suite passes (4 slow tests skipped)
• all pre-commit checks pass
• GitNexus change analysis: low risk, only the intended Gateway auto-config and regression-test symbols changed