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/validate.py

@@ -58,7 +58,7 @@ def validate_decode(case: dict, path: Path) -> list[str]:
 
     # Run decoder — must not crash
     try:
-        raw_output = decode_meshcore_packet(packet)
+        raw_output, _warnings = decode_meshcore_packet(packet)
     except Exception as e:
         errors.append(f"Decoder crashed: {e}")
         return errors