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>

tools/build-image.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Build the boxsim image, staging the Visaton library and parts catalogue.
+#
+# Both live outside this repository, so they are copied into a staging directory
+# inside the build context first. Nothing proprietary is baked in: the Visaton
+# library is our own, and the catalogue is a plain text parts list.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$ROOT"
+
+LIB_DIR="${BOXSIM_KICAD_LIB_DIR:-/home/magdev/workspaces/other/kicad-visaton-lib}"
+CATALOG="${BOXSIM_CATALOG:-$ROOT/boxsim_dl/BoxSim 2.1 DE (Stand 12.2025)/Weichenbauteile/Visatonteile.txt}"
+IMAGE="${IMAGE:-boxsim:0.1.0}"
+
+SYM="$LIB_DIR/symbols/Visaton.kicad_sym"
+FP="$LIB_DIR/footprints/Visaton.pretty"
+
+# A missing input must stop the build, never silently produce an image without
+# its library.
+for path in "$SYM" "$FP" "$CATALOG"; do
+    if [[ ! -e "$path" ]]; then
+        echo "build-image: required input missing: $path" >&2
+        exit 1
+    fi
+done
+
+# The library must already be in the current KiCad format, otherwise the symbols
+# cannot be embedded into a KiCad 10 schematic.
+version="$(sed -n 's/.*(version \([0-9]\+\)).*/\1/p' "$SYM" | head -1)"
+if [[ -z "$version" || "$version" -lt 20211014 ]]; then
+    echo "build-image: $SYM is in the legacy format (version ${version:-none})." >&2
+    echo "             Run: kicad-cli sym upgrade --force '$SYM'" >&2
+    exit 1
+fi
+
+rm -rf stage
+mkdir -p stage
+cp "$SYM" stage/Visaton.kicad_sym
+cp "$CATALOG" stage/Visatonteile.txt
+cp -r "$FP" stage/footprints
+
+echo "staged: symbol library (version $version), catalogue, $(ls stage/footprints | wc -l) footprints"
+
+docker build -t "$IMAGE" .
+rm -rf stage
+
+echo "built $IMAGE"