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/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