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/path_3byte_hash.json

@@ -0,0 +1,29 @@
+{
+  "description": "PATH flood, 1 hop with 3-byte hash",
+  "tags": [
+    "path",
+    "flood",
+    "3byte_hash",
+    "encrypted"
+  ],
+  "packet_hex": "21811afff3e6e92e4700000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
+  "valid": true,
+  "expected": {
+    "route_type": "flood",
+    "payload_type": "PATH",
+    "payload_ver": 0,
+    "transport_codes": null,
+    "path": {
+      "hash_size": 3,
+      "hops": [
+        "1afff3"
+      ]
+    },
+    "payload": {
+      "dst": "e6",
+      "src": "e9",
+      "mac": "2e47",
+      "ciphertext_len": 46
+    }
+  }
+}