Add cached gplan.so build: 30min first time, 3sec after

- Dockerfile.gplan: pre-built armv7 image with cbang + camotics objects
- build-gplan.sh: relinks against Python 3.5m in ~3sec
- Pi Python 3.5 headers cached in .pi/pi-python35.tar.gz (gitignored)

.pi/build-gplan.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Build gplan.so for the Onefinity Pi (armv7l, Python 3.5)
+#
+# First run:   ~30min (builds the Docker image with pre-compiled cbang/camotics)
+# After that:  ~2sec  (just relinks against Python 3.5m)
+#
+# Prerequisites:
+#   - Docker with QEMU binfmt support (default on Docker Desktop)
+#   - Python 3.5 headers from the Pi in .pi/pi-python35.tar.gz
+#     Grab once: ssh bbmc@10.1.10.55 'tar czf - /usr/include/python3.5m \
+#       /usr/lib/arm-linux-gnueabihf/libpython3.5m.so*' > .pi/pi-python35.tar.gz
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
+IMAGE="onefin-gplan"
+HEADERS="$SCRIPT_DIR/pi-python35.tar.gz"
+OUTPUT="$PROJECT_DIR/src/py/camotics/gplan.so"
+
+# Check for Python 3.5 headers
+if [[ ! -f "$HEADERS" ]]; then
+    echo "Python 3.5 headers not found at $HEADERS"
+    echo "Fetching from Pi..."
+    ssh bbmc@10.1.10.55 'tar czf - /usr/include/python3.5m \
+      /usr/lib/arm-linux-gnueabihf/libpython3.5m.so*' > "$HEADERS"
+fi
+
+# Build image if needed (one-time, ~30min)
+if ! docker image inspect "$IMAGE" >/dev/null 2>&1; then
+    echo "Building $IMAGE Docker image (one-time, ~30min under QEMU)..."
+    docker build --platform linux/arm/v7 -t "$IMAGE" -f "$SCRIPT_DIR/Dockerfile.gplan" "$SCRIPT_DIR"
+fi
+
+# Relink gplan.so against Python 3.5m (~2sec)
+echo "Linking gplan.so against Python 3.5m..."
+docker run --rm --platform linux/arm/v7 \
+    -v "$HEADERS:/tmp/pi-python35.tar.gz:ro" \
+    -v "$PROJECT_DIR:/workspace" \
+    "$IMAGE" bash -c '
+    tar xzf /tmp/pi-python35.tar.gz -C /
+    ln -sf /usr/lib/arm-linux-gnueabihf/libpython3.5m.so.1.0 \
+           /usr/lib/arm-linux-gnueabihf/libpython3.5m.so
+
+    g++ -o /workspace/src/py/camotics/gplan.so \
+      -Wl,--as-needed -Wl,-s -Wl,-x -Wl,--gc-sections -pthread -shared \
+      build/gplan.os -L/opt/cbang/lib \
+      build/libCAMoticsPy.a build/libCAMotics.a build/libDXF.a \
+      build/libSTL.a build/libGCode.a \
+      -lstdc++ -lutil -lm -ldl -lz -lcbang -lcbang-boost \
+      -lssl -lcrypto -llz4 -lexpat -lbz2 -lcrypt -lpthread \
+      -lpython3.5m build/dxflib/libdxflib.a
+
+    file /workspace/src/py/camotics/gplan.so
+    readelf -d /workspace/src/py/camotics/gplan.so | grep python
+'
+
+echo "✓ Built: $OUTPUT"