Add use_aead field to ChannelSettings for AES-CCM channel encryption

[matutetandil]

Summary

• Add bool use_aead = 8 to the ChannelSettings message in channel.proto
• Enables optional AES-CCM authenticated encryption on PSK channels
• When true, messages include a 12-byte auth tag preventing forgery and bit-flipping
• Defaults to false (standard AES-CTR encryption, no behavior change)

Context

This field is required by the firmware implementation of AEAD for PSK channels (firmware#4030). Design validated by @pqcfox (applied cryptographer).

Parameters: 12-byte auth tag, no CTR fallback on AEAD-enabled channels, experimental labeling.

The companion firmware PR will be submitted against meshtastic/firmware once this protobuf change is available.

Test plan

• Protobuf compiles with protoc without errors
• Field number 8 does not conflict with existing fields (next after module_settings = 7)
• Default false preserves backward compatibility — existing nodes unaffected
• Firmware PR validates encrypt/decrypt round-trip with 10 unit test sub-cases

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