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/generate.py

@@ -94,7 +94,7 @@ def advert_appdata(draw):
             name = data[8 if has_loc else 0:].decode("ascii", errors="replace")
 
     app_data = bytes([flags]) + data
-    node_label = NODE_TYPES.get(node_type, f"type={node_type}")
+    node_label = NODE_TYPES.get(node_type, f"0x{node_type:02x}")
     return app_data, {"flags": flags, "node_type": node_label, "location": loc, "name": name}
 
 
@@ -158,7 +158,7 @@ def valid_encrypted_peer_packet(draw):
     dst = draw(st.integers(min_value=0, max_value=255))
     src = draw(st.integers(min_value=0, max_value=255))
     mac = draw(st.binary(min_size=2, max_size=2))
-    ct_len = draw(st.integers(min_value=0, max_value=64))
+    ct_len = draw(st.integers(min_value=16, max_value=64).filter(lambda n: n % 16 == 0))
     ct = draw(st.binary(min_size=ct_len, max_size=ct_len))
 
     hdr = bytes([mc_header(ptype, route_type)])
@@ -188,7 +188,7 @@ def valid_group_packet(draw):
     hs, hops = draw(path_strategy())
     ch = draw(st.integers(min_value=0, max_value=255))
     mac = draw(st.binary(min_size=2, max_size=2))
-    ct_len = draw(st.integers(min_value=0, max_value=64))
+    ct_len = draw(st.integers(min_value=16, max_value=64).filter(lambda n: n % 16 == 0))
     ct = draw(st.binary(min_size=ct_len, max_size=ct_len))
 
     hdr = bytes([mc_header(ptype, route_type)])
@@ -220,7 +220,7 @@ def invalid_packet(draw):
         pkt, _ = draw(valid_ack_packet())
         cut = draw(st.integers(min_value=0, max_value=max(0, len(pkt) - 2)))
         packet = pkt[:cut]
-        if len(packet) < 2:
+        if len(packet) < 3:
             error = "too_short"
         else:
             error = "bad_framing"
@@ -230,12 +230,18 @@ def invalid_packet(draw):
         ptype = draw(st.sampled_from(list(PAYLOAD_TYPES.keys())))
         hdr = bytes([mc_header(ptype, route_type)])
         # path_len claiming huge path
-        bad_pl = draw(st.integers(min_value=0xC0, max_value=0xFF))
+        # Use hash_size_code 3 (reserved, 0xC0-0xFF) or valid codes with huge counts
+        bad_pl = draw(st.sampled_from([
+            *range(0xC0, 0x100),  # reserved hash_size_code 3
+            0x3F,  # 63 hops × 1 byte = 63 (valid encoding, will overflow data)
+            0x60,  # 32 hops × 2 bytes = 64 (valid encoding, will overflow data)
+            0x95,  # 21 hops × 3 bytes = 63 (valid encoding, will overflow data)
+        ]))
         tail = draw(st.binary(min_size=1, max_size=20))
         packet = hdr + bytes([bad_pl]) + tail
         error = "bad_framing"
     else:  # random
-        packet = draw(st.binary(min_size=2, max_size=50))
+        packet = draw(st.binary(min_size=3, max_size=50))
         error = "not_meshcore"
 
     header = packet[0] if packet else 0