feat: MeshCore decoder, Watchman, test corpus, sparkline TX indicators

Firmware:
- fix(protocol): SNR type i8→i16 to match lora_phy PacketStatus
- fix(display): combine splash + waiting into one screen
- fix(display): move TX power to row 0 right-justified
- feat(display): sparkline distinguishes TX from RX — TX bars are dotted
  (alternating pixel rows), RX bars are solid. TX-only slots show a short
  marker. TX takes precedence over RX in the same time slot.

Host tool (tools/meshcore_rx.py):
- feat: full MeshCore packet decoder with colored ANSI output
- feat: GRP_TXT decryption using 276 channels from channels.csv
- feat: Watchman loop/collision reporter to #watchman
- feat: ADVERT-informed node registry with 12h TTL persistence
- feat: transport code fallback for older MeshCore firmware
- fix: robust error handling for long-running operation
- fix: collision detection uses evidence-based reasoning from advert cache

Channel directory (tools/channels.csv): 276 channels

Test corpus (corpus/): JSON Schema, 28 hand-crafted vectors, Hypothesis generator

src/radio/task.rs

@@ -50,7 +50,7 @@ pub async fn radio_task(
                         Ok((len, pkt_status)) => {
                             state.rx_count = state.rx_count.wrapping_add(1);
                             let rssi = pkt_status.rssi;
-                            let snr = pkt_status.snr as i8;
+                            let snr = pkt_status.snr;
                             state.last_rssi = Some(rssi);
                             state.last_snr = Some(snr);
                             status.sender().send(state.clone());