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>

docs/plans/2026-03-18-refactor-progress.md

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+# pm3py Refactor Progress
+
+**Design:** [2026-03-18-refactor-design.md](2026-03-18-refactor-design.md)
+**Branch:** `refactor/core-package`
+**Started:** 2026-03-18
+
+## Phase 1 — Create `core/`, normalize names, fix imports
+
+**Status: Complete**
+
+### What was done
+
+- Created `pm3py/core/` sub-package
+- Moved all source modules into `core/`:
+  - `protocol.py`, `transport.py`, `client.py`, `hw.py`, `hf.py`, `lf.py` — moved as-is
+  - `hf_14a.py` → `hf_iso14a.py` (renamed)
+  - `hf_15.py` → `hf_iso15.py` (renamed)
+  - `hf_mf.py` → `hf_mfc.py` (renamed)
+- Normalized client attributes:
+  - `hf.a14` → `hf.iso14a`
+  - `hf.mf` → `hf.mfc`
+  - `hf.iso15` — unchanged
+- Created `core/__init__.py` re-exporting: `Proxmark3`, `FirmwareInfo`, `PM3Transport`, `PM3Error`, `PM3Response`, `PM3Status`, `Cmd`
+- Updated `pm3py/__init__.py` to import from `.core.*`
+- Updated all 10 test files (imports + attribute references)
+- **30/30 tests passing**
+
+### Backward compatibility
+
+- `from pm3py import Proxmark3` still works (re-exported)
+- **Breaking:** `hf.a14` → `hf.iso14a`, `hf.mf` → `hf.mfc`
+- **Breaking:** Direct imports like `from pm3py.hf_14a import ...` → `from pm3py.core.hf_iso14a import ...`
+
+## Phase 2 — Extract `sniff/`
+
+**Status: Deferred — no sniff infrastructure exists yet**
+
+`hf_iso15.py` is 94 lines of clean reader commands + thin `sniff()` firmware call. The trace parsing, 15693 decoders, NDEF annotation, and ANSI formatting described in the design doc haven't been written yet. Nothing to extract.
+
+Will revisit when sniff infrastructure is implemented.
+
+## Phase 3 — Scaffold `transponders/`
+
+**Status: Not started — models live in sim worktree**
+
+Transponder models exist in `.worktrees/sim-framework/` but haven't been merged to master. Scaffold when ready to migrate.
+
+## Phase 4 — Scaffold `reader/` and `sim/`
+
+**Status: Not started**
+
+Depends on Phase 3 and sim worktree merge.