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>

Dockerfile

@@ -0,0 +1,41 @@
+# boxsim: decode Boxsim projects and generate KiCad projects from them.
+#
+# The image deliberately contains no KiCad and no Boxsim.exe:
+#   - KiCad 10 is a hard dependency of the *verification* gate only, and stays
+#     on the host. decode/extract/netlist/build all work without it.
+#   - The A2F codec is pure Python, so Boxsim's proprietary binary is not needed.
+#
+# Build via tools/build-image.sh, which stages the Visaton library and the
+# Boxsim parts catalogue into the build context.
+
+FROM python:3.13-slim AS base
+
+LABEL org.opencontainers.image.title="boxsim" \
+      org.opencontainers.image.description="Boxsim project decoder and KiCad project generator" \
+      org.opencontainers.image.source="https://src.bundespruefstelle.ch/boxdev/re-boxsim" \
+      org.opencontainers.image.licenses="MIT"
+
+WORKDIR /opt/boxsim
+
+COPY pyproject.toml README.md ./
+COPY boxsim ./boxsim
+
+# Staged by tools/build-image.sh: the upgraded Visaton symbol library and the
+# Visaton parts catalogue shipped with Boxsim.
+COPY stage/Visaton.kicad_sym ./boxsim/data/Visaton.kicad_sym
+COPY stage/Visatonteile.txt ./boxsim/data/Visatonteile.txt
+COPY stage/footprints ./boxsim/data/footprints
+
+COPY tools/selfcheck.py ./tools/selfcheck.py
+
+RUN pip install --no-cache-dir .
+
+ENV BOXSIM_CATALOG=/opt/boxsim/boxsim/data/Visatonteile.txt \
+    BOXSIM_KICAD_LIB=/opt/boxsim/boxsim/data/Visaton.kicad_sym
+
+# Refuse to produce an image whose bundled library does not cover the catalogue.
+RUN python tools/selfcheck.py
+
+WORKDIR /work
+ENTRYPOINT ["boxsim"]
+CMD ["--help"]