Experimental lr1121 generic support

[aslater3]

Summary

This PR adds LR1121-specific TX power handling for multi-band operation by enforcing different maximum TX power limits based on active RF band.

What Changed

• Updated CustomLR1121 to track the currently configured band from setFrequency(...).
• Added LR1121-specific TX power clamping in both setOutputPower(...) overloads:
  • 2.4 GHz band: max 20 dBm
  • sub-GHz band: max 22 dBm
• Kept changes scoped to CustomLR1121 so non-LR1121 radios and boards are unaffected.

Why

This addresses maintainer feedback that LR1121 needs special handling because it supports multiple radios with different TX power limits by band.

Scope / Compatibility

• No behavior change for non-LR1121 targets.
• No board-level logic changes required for existing non-LR1121 variants.

Testing

• Code-level validation only in this PR.
• Full hardware/runtime validation should be done on LR1121 targets in both sub-GHz and 2.4 GHz configurations. Tested locally on subGhz in repeater and companion mode.