refactor: move modules into core/ sub-package, normalize naming

Phase 1 of package refactor. Moves all source modules into pm3py/core/
with file renames (hf_14a→hf_iso14a, hf_15→hf_iso15, hf_mf→hf_mfc)
and client attribute normalization (hf.a14→hf.iso14a, hf.mf→hf.mfc).
pm3py/__init__.py re-exports from core for backward compat.

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

tests/test_hf_15.py

@@ -1,8 +1,8 @@
 import asyncio
 from unittest.mock import AsyncMock
-from pm3py.protocol import Cmd
-from pm3py.transport import PM3Response
-from pm3py.hf_15 import HF15Commands
+from pm3py.core.protocol import Cmd
+from pm3py.core.transport import PM3Response
+from pm3py.core.hf_iso15 import HF15Commands
 
 def test_15_scan():
     t = AsyncMock()