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.

Makefile

@@ -68,7 +68,11 @@ update: pkg
 
 build/templates.pug: $(TEMPLS)
 	mkdir -p build
-	cat $(TEMPLS) >$@
+	# Use awk to ensure each template is followed by a newline so the
+	# next file's first line never gets glued onto the previous file's
+	# last line (some templates ship without a trailing newline, which
+	# would produce subtle Pug parse failures).
+	awk 'FNR==1 && NR>1 {print ""} {print} END{print ""}' $(TEMPLS) >$@
 
 node_modules: package.json
 	npm install && touch node_modules