Add explicit local Dig2Go propagation trigger

[theysayheygreg]

Summary

• add a USB-local P! command for the propagation-capable Dig2Go build
• route it through the existing exact-target FleetUpdateOffer command path
• preserve bare P power-save and structured P<offer> behavior
• document and regress the local trigger contract

Verification

• ./test/tubes_mesh/run.sh
• ordinary Dig2Go, P2P Dig2Go, Athom C3, and S3 carrier v51 builds
• no hardware flashed by this change

This repairs the current Easy Flash Electron handoff: v51 intentionally removed the old Q source-selection command, so the app now sends P! and waits for FLEET_RX propagation=accepted mode=command.

[theysayheygreg]

Superseded by a clean v52 proposal that promotes propagation into the canonical Dig2Go build. Closing this narrow v51 P! patch so the capability and release boundary land together.