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)

.gitignore

@@ -37,3 +37,4 @@ src/py/camotics/gplan.so
 src/avr/emu/bbemu
 src/avr/emu/build/
 
+.pi/pi-python35.tar.gz

.pi/Dockerfile.gplan

@@ -0,0 +1,40 @@
+# Pre-built armv7 environment for gplan.so
+# Build once:  docker build --platform linux/arm/v7 -t onefin-gplan -f .pi/Dockerfile.gplan .pi/
+# Then use:    .pi/build-gplan.sh
+FROM debian:bullseye
+
+RUN apt-get update -qq && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
+      build-essential scons git ca-certificates python3-dev \
+      libssl-dev libexpat1-dev libbz2-dev liblz4-dev zlib1g-dev perl file && \
+    rm -rf /var/lib/apt/lists/*
+
+# Clone and build cbang (static lib, ~5min under QEMU)
+RUN mkdir -p /opt/cbang && cd /opt/cbang && git init -q && \
+    git remote add origin https://github.com/CauldronDevelopmentLLC/cbang && \
+    git fetch --depth 1 -q origin 18f1e963107ef26abe750c023355a5c40dd07853 && \
+    git reset --hard FETCH_HEAD -q && \
+    scons -j2 disable_local="re2 libevent" && \
+    rm -rf .git build/dep
+
+# Clone and patch camotics (source only, don't compile yet — that depends on workspace)
+RUN mkdir -p /opt/camotics && cd /opt/camotics && git init -q && \
+    git remote add origin https://github.com/CauldronDevelopmentLLC/camotics && \
+    git fetch --depth 1 -q origin ec876c80d20fc19837133087cef0c447df5a939d && \
+    git reset --hard FETCH_HEAD -q && \
+    mkdir -p build && touch build/version.txt && \
+    P="src/gcode/plan" && \
+    for F in LineCommand.cpp LinePlanner.cpp; do \
+      for V in maxVel maxJerk maxAccel; do \
+        perl -i -0pe "s/(fabs\((config\.$V\[axis\]) \/ unit\[axis\]\));/std::min(\2, \1);/gm" $P/$F; \
+      done; \
+    done && \
+    rm -rf .git
+
+# Pre-compile camotics objects (the slow part, ~20min under QEMU)
+ENV CBANG_HOME=/opt/cbang
+RUN cd /opt/camotics && scons -j2 gplan.so with_gui=0 with_tpl=0 && \
+    rm -f gplan.so  # remove the python3.9-linked one, we relink at build time
+
+ENV CBANG_HOME=/opt/cbang
+WORKDIR /opt/camotics

.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"