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_crc.py

@@ -1,4 +1,4 @@
-from pm3py.protocol import crc16_a
+from pm3py.core.protocol import crc16_a
 
 def test_crc16_a_empty():
     assert crc16_a(b"") == 0x6363  # init 0xC6C6 reflected = 0x6363