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

corpus/hand/response_valid_1hop.json

@@ -0,0 +1,14 @@
+{
+  "description": "RESPONSE flood, 1 hop (real capture — same message as corrupted variants)",
+  "tags": ["response", "flood", "path", "real_capture"],
+  "packet_hex": "0501ac5bd17863f6ec7bbbd9bf8e5ea7d44b6a0e8f6318",
+  "valid": true,
+  "expected": {
+    "route_type": "flood",
+    "payload_type": "RESPONSE",
+    "payload_ver": 0,
+    "transport_codes": null,
+    "path": {"hash_size": 1, "hops": ["ac"]},
+    "payload": {"dst": "5b", "src": "d1", "mac": "7863", "ciphertext_len": 16}
+  }
+}