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>