Add boxsim CLI: pure Python codec, netlist extraction, KiCad project generation

Replaces the bpj2ini.py prototype with a packaged tool. The A2F codec is now
reimplemented in Python, so neither Boxsim.exe nor a CPU emulator is needed to
decode a container; the emulator survives as a development-only oracle that the
codec is differentially tested against.

Verified:
- 134 containers decode byte-identically to Boxsim's own decoder, 270 pre-2.0
  files rejected by both implementations (tools/difftest.py --self-test)
- Default.BPJ: 22 components, 14 nets, no dangling terminal
- 22 of 22 parts resolve to library symbols; one series inferred from a
  sibling, two DC resistance deviations reported
- ERC clean and the netlist KiCad extracts from the generated sheet is an
  identical partition to the one derived from the Boxsim grid
- container build byte-identical to the host build
- 32 tests passing, ruff clean

Decode time went 69 s -> 14.5 s (binding emulator hooks to trap addresses)
-> 1.6 s (pure Python).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_codec.py

@@ -0,0 +1,79 @@
+"""Codec tests. The heavy differential sweep against Boxsim.exe lives in
+tools/difftest.py; these guard regressions against the digests it recorded.
+"""
+
+from __future__ import annotations
+
+import hashlib
+from pathlib import Path
+
+import pytest
+
+from boxsim import a2f, container
+from boxsim.krypt import KryptPRNG
+
+ROOT = Path(__file__).resolve().parent.parent
+
+
+def test_keystream_is_stable():
+    """Regression guard on the keystream every Boxsim file is scrambled with.
+
+    The value is not independently derived, it is the keystream under which all
+    134 containers in the distribution decode byte-identically to Boxsim's own
+    decoder (see tools/difftest.py). Its worth here is catching an accidental
+    change to the generator, not proving the generator correct.
+    """
+    ks = KryptPRNG().keystream(8)
+    assert ks.hex() == "ebe14331cb3183e1"
+
+
+def test_rejects_foreign_file():
+    with pytest.raises(container.ContainerError, match="not a MyIni2CK file"):
+        container.decode(b"not a boxsim file at all")
+
+
+def test_truncated_stream_fails_loudly():
+    raw = (ROOT / "boxsim_dl" / "BoxSim 2.1 DE (Stand 12.2025)"
+           / "Projekte" / "Default.BPJ")
+    if not raw.exists():
+        pytest.skip("distribution not present")
+    data = raw.read_bytes()
+    with pytest.raises(a2f.A2FError):
+        container.decode(data[: len(data) // 2])
+
+
+def test_padding_allowance_is_bounded():
+    # A stream of zeros must not decode forever; it has to hit the limit.
+    with pytest.raises(a2f.A2FError):
+        a2f.expand(b"MyIni2CK", skip_magic=8)
+
+
+@pytest.mark.needs_dist
+def test_matches_oracle_digests(manifest: dict):
+    """Every container the oracle decoded must still decode identically."""
+    files = manifest["files"]
+    assert files, "manifest has no entries"
+    checked = 0
+    for rel, expected in files.items():
+        path = ROOT / rel
+        if not path.exists():
+            pytest.fail(f"manifest references missing file {rel}")
+        payload = container.decode(path.read_bytes())
+        digest = hashlib.sha256(payload).hexdigest()
+        assert digest == expected["sha256"], f"{rel} digest changed"
+        assert len(payload) == expected["len"], f"{rel} length changed"
+        checked += 1
+    assert checked == len(files)
+
+
+@pytest.mark.needs_dist
+def test_pre20_containers_are_rejected(manifest: dict):
+    """The BoxSim 1.2 files use an older container and must stay rejected."""
+    rejected = manifest["rejected_not_myini2ck"]
+    assert rejected, "manifest recorded no rejected files"
+    for rel in rejected:
+        path = ROOT / rel
+        if not path.exists():
+            continue
+        with pytest.raises(container.ContainerError):
+            container.decode(path.read_bytes())