[2.8.0] Configure packet authenticity policy

[RCGV1]

Description

Add the Packet authenticity selector specified in meshtastic/design#121 to the Apple client Security configuration UI on iOS, iPadOS, and macOS.

Expose the device SecurityConfig.packet_signature_policy using native controls and the shared copy:

• Compatible — Accept unsigned
• Balanced — Prefer signed (recommended/default)
• Strict — Require signed

Strict must present a confirmation explaining that older nodes, licensed/ham nodes without PKI keys, and oversized unsigned broadcasts may disappear. PKI-encrypted direct messages remain unaffected.

Acceptance criteria

• Read/write the firmware field; do not add an app-local packet filter.
• Gate the control on firmware/protobuf support and render the zero/default as Balanced.
• Preserve shield=authentic and lock=private semantics from design#113.
• Add unit/UI coverage for enum mapping, configuration save, Strict confirmation/cancel, and unavailable firmware.
• Provide reviewed light/dark screenshots for all values, confirmation, and iPhone/iPad/macOS layouts where the settings surface differs.
• Keep the implementation PR draft until device configuration round-trip and receive filtering are verified against hardware.

Participation: I am willing to submit a pull request for this issue.

[RCGV1]

Scope update from design#121: Strict is authenticated-only for every decryptable remote packet, not broadcast-only. New firmware signs signable non-PKI unicast/control packets as well as broadcasts; Strict accepts verified XEdDSA or successfully authenticated PKI and rejects other decrypted traffic regardless of portnum/destination. Use Strict supporting copy such as “Only show and process cryptographically authenticated mesh packets. Older nodes and oversized packets may disappear.”.

[RCGV1]

Scope correction from design#121: this feature does not create a new authentication version and does not broaden normal-mode outbound signing. It only configures firmware receive enforcement using Meshtastic's existing XEdDSA/PKI results. Strict drops every decryptable remote packet that is not authenticated by an existing valid XEdDSA signature or successful existing PKI decryption before local storage/client/module/MQTT processing. Existing normal-mode broadcast signing and PKI direct-message behavior remain unchanged. Authenticated plaintext licensed/ham broadcasts and direct messages are tracked separately in design#122.