feat: Meshtastic TX — device announces as a named node (messageable)

The device now transmits a NodeInfo (User protobuf: id/long_name/short_name)
on the LongFast primary channel every 60s, so it appears in other Meshtastic
clients as 'Reticutasticore' / 'RTC'. NodeNum derived from the MAC
(!4359d2d0). No public key advertised, so DMs to us fall back to channel
encryption — which the existing RX path already decodes and displays.

Confirmed on hardware: announce tx=0 (success) on the first T-window.

Also spec the planned message-received animation (upside-down spinning
smiley + lightning bolts -> Meshtastic-style modal) in the README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

firmware/README.md

@@ -53,6 +53,13 @@ pio run -t upload --upload-port /dev/cu.usbserial-0001   # flash (adjust port)
 pio device monitor -b 115200                     # watch the scan
 ```
 
+## Planned: message-received animation
+
+When a text message arrives, play a short intro before showing it: the striped
+smiley spins **upside-down / rotated**, little lightning bolts flash, then a
+Meshtastic-style **message modal** slides in with the sender + text. (Spec'd,
+not yet built.)
+
 ## Roadmap from here
 
 1. **RX decode per protocol** — turn logged raw packets into `{sender, text}`