build: document Pi firmware build/flash + gplan.so cross-build via Stretch Docker

- .pi/BUILD.md: end-to-end macOS dev workflow, deploy paths, dphys-swapfile vs fstab, troubleshooting.
- .pi/Dockerfile.gplan + build-gplan.sh: rebuild gplan.so from source on Raspbian Stretch (Bullseye is too new for the toolchain).
- Makefile: ensure trailing newline between concatenated pug templates so Pug doesn't glue file boundaries together.

.pi/Dockerfile.gplan

@@ -0,0 +1,48 @@
+# Raspbian Stretch armhf build environment for gplan.so
+# Matches the Pi exactly: GCC 6.3, Python 3.5, GLIBC 2.24
+#
+# Build image:  docker build -t onefin-gplan -f .pi/Dockerfile.gplan .pi/
+# Build gplan:  .pi/build-gplan.sh
+FROM balenalib/raspberry-pi-debian:stretch
+
+# Fix repos to use archived Raspbian mirrors
+RUN echo "deb http://legacy.raspbian.org/raspbian/ stretch main contrib non-free rpi" \
+      > /etc/apt/sources.list && \
+    rm -f /etc/apt/sources.list.d/*.list
+
+RUN apt-get -o Acquire::Check-Valid-Until=false \
+      -o Acquire::AllowInsecureRepositories=true update && \
+    apt-get -o Acquire::Check-Valid-Until=false --allow-unauthenticated \
+      install -y --no-install-recommends \
+      build-essential python3-dev scons git ca-certificates \
+      libssl-dev libexpat1-dev libbz2-dev liblz4-dev zlib1g-dev perl file && \
+    rm -rf /var/lib/apt/lists/*
+
+# Clone and build cbang
+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, patch, and build camotics/gplan
+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
+
+ENV CBANG_HOME=/opt/cbang
+
+# Pre-compile everything including gplan.so
+RUN cd /opt/camotics && scons -j2 gplan.so with_gui=0 with_tpl=0
+
+WORKDIR /opt/camotics