fix: MeshCore spec compliance audit and packet quality warnings

Audited against meshcore-spec and fixed all compliance gaps:

- Fix trace hash size encoding: use 1<<(flags&3) not (flags&3)+1
- Reject reserved path hash_size_code 3 per spec Section 3
- Minimum packet size 3 bytes (was 2)
- Add MAX_PACKET_PAYLOAD (184) and min payload (1) validation
- Tighten payload minimums: ANON_REQ≥51, encrypted≥20, group≥19,
  multipart≥2
- Add node_type 0 ("none") to NODE_TYPES
- Fix node_type fallback display (was "node=type=14", now "node=0x0e")

Add packet quality warnings for likely-corrupt frames:
- decode_meshcore_packet returns (str, list[str]) with quality warnings
- ADVERT: flag bad node_type, bad UTF-8 in name, future/bad timestamps
- Combine content warnings with low-SNR signal at display layer
- Warnings stack: one flag = maybe, multiple = high confidence corrupt

Update test corpus and generators for tightened validation rules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/meshcore/corpus/hand/bad_path_len.json

@@ -1,11 +1,11 @@
 {
-  "description": "TXT_MSG flood, path_len claims 33 hops (4B hash) but data is too short",
+  "description": "TXT_MSG flood, path_len claims 33 hops (2B hash) but data is too short",
   "tags": [
     "invalid",
     "bad_framing",
     "path"
   ],
-  "packet_hex": "09e1000000000000000000000000000000000000000000000000",
+  "packet_hex": "0961000000000000000000000000000000000000000000000000",
   "valid": false,
   "expected": {
     "route_type": "flood",

examples/meshcore/corpus/hand/bad_path_len_reserved_hash_size.json

@@ -0,0 +1,16 @@
+{
+  "description": "TXT_MSG flood, path_len has reserved hash_size_code 3",
+  "tags": [
+    "invalid",
+    "bad_framing",
+    "path",
+    "reserved_hash_size"
+  ],
+  "packet_hex": "09c1000000000000000000000000000000000000000000000000",
+  "valid": false,
+  "expected": {
+    "route_type": "flood",
+    "payload_type": "TXT_MSG",
+    "error": "bad_framing"
+  }
+}

examples/meshcore/corpus/hand/path_4byte_hash.json

@@ -1,29 +1,17 @@
 {
-  "description": "PATH flood, 1 hop with 4-byte hash",
+  "description": "PATH flood, 4-byte hash (reserved hash_size_code 3, invalid)",
   "tags": [
+    "invalid",
     "path",
     "flood",
     "4byte_hash",
-    "encrypted"
+    "reserved_hash_size"
   ],
   "packet_hex": "21c1aabbccdde6e92e4700000000000000000000000000000000",
-  "valid": true,
+  "valid": false,
   "expected": {
     "route_type": "flood",
     "payload_type": "PATH",
-    "payload_ver": 0,
-    "transport_codes": null,
-    "path": {
-      "hash_size": 4,
-      "hops": [
-        "aabbccdd"
-      ]
-    },
-    "payload": {
-      "dst": "e6",
-      "src": "e9",
-      "mac": "2e47",
-      "ciphertext_len": 16
-    }
+    "error": "bad_framing"
   }
 }

examples/meshcore/corpus/hand/too_short_2bytes.json

@@ -0,0 +1,14 @@
+{
+  "description": "Packet too short (2 bytes, need >= 3)",
+  "tags": [
+    "invalid",
+    "too_short"
+  ],
+  "packet_hex": "1100",
+  "valid": false,
+  "expected": {
+    "route_type": "flood",
+    "payload_type": "ADVERT",
+    "error": "too_short"
+  }
+}

examples/meshcore/corpus/hand/trace_4byte_hash.json

@@ -0,0 +1,24 @@
+{
+  "description": "TRACE flood with 4-byte trace hashes (flags=0x02, hash_size=4)",
+  "tags": [
+    "trace",
+    "flood",
+    "4byte_hash"
+  ],
+  "packet_hex": "2500ddccbbaa4433221102deadbeefcafebabe",
+  "valid": true,
+  "expected": {
+    "route_type": "flood",
+    "payload_type": "TRACE",
+    "payload_ver": 0,
+    "transport_codes": null,
+    "path": {
+      "hash_size": 1,
+      "hops": []
+    },
+    "payload": {
+      "trace_tag": 2864434397,
+      "trace_hops": 2
+    }
+  }
+}