Merge branch 'refactor/core-package' into master

Merges the package refactor (core/ sub-package, normalized naming,
scaffolded sub-packages) while preserving all in-progress sniff
infrastructure. Git rename detection carried WIP changes to new paths.

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

tests/test_hf_15.py

@@ -1,15 +1,14 @@
 import asyncio
 import struct
 from unittest.mock import AsyncMock
-from pm3py.protocol import Cmd
-from pm3py.transport import PM3Response
-from pm3py.hf_15 import (
+from pm3py.core.protocol import Cmd
+from pm3py.core.transport import PM3Response
+from pm3py.core.hf_iso15 import (
     HF15Commands, parse_tracelog, TRACELOG_HDR_SIZE,
     decode_15693_request, decode_15693_response, decode_15693,
     format_sniff_line, decode_ndef_annotation,
 )
 
-
 def test_15_scan():
     t = AsyncMock()
     iso15 = HF15Commands(t)