Add multiarch build setup and build documentation

- Add `make setup` target to register qemu and create a multiplatform
  buildx builder for universal (x86_64 + aarch64) packages
- Use named builder with --progress=plain to avoid flickering output
- Conditionally depend on arch-specific Docker tars so single-arch
  builds don't require the other image
- Document prerequisites, build targets, and overrides in README

Makefile

@@ -6,13 +6,16 @@ TS_FILES := $(shell find ./scripts -name \*.ts 2>/dev/null)
 # Where KamadoPool source lives (buildx passes it as a named context).
 KAMADO_SRC ?= ../KamadoPool
 
+# Buildx builder name. Must support linux/amd64 and linux/arm64.
+BUILDER ?= kamado-multiarch
+
 .DELETE_ON_ERROR:
 
 # Default: build both arches and verify a universal package.
 all: verify
 
-# Single-arch convenience targets — much faster if you don't have
-# qemu-user-static registered for cross-arch emulation.
+# Single-arch convenience targets — much faster when you don't need
+# cross-arch emulation.
 x86: ARCH = x86_64
 x86:
 	@rm -f docker-images/aarch64.tar
@@ -23,6 +26,14 @@ arm:
 	@rm -f docker-images/x86_64.tar
 	$(MAKE) ARCH=aarch64
 
+# Register qemu for cross-arch builds and create a multiplatform builder.
+setup:
+	docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
+	-docker buildx rm $(BUILDER) 2>/dev/null
+	docker buildx create --name $(BUILDER) --platform linux/amd64,linux/arm64 --use
+	docker buildx inspect --bootstrap $(BUILDER)
+	@echo "Ready. 'make' will now build universal packages."
+
 verify: $(PKG_ID).s9pk
 	start-sdk verify s9pk $(PKG_ID).s9pk
 	@echo "Package: $(PKG_ID).s9pk ($(shell du -h $(PKG_ID).s9pk | cut -f1))"
@@ -38,7 +49,7 @@ scripts/embassy.js: $(TS_FILES)
 docker-images/x86_64.tar: Dockerfile docker_entrypoint.sh
 ifneq ($(ARCH),aarch64)
 	mkdir -p docker-images
-	docker buildx build \
+	docker buildx build --builder $(BUILDER) --progress=plain \
 		--tag start9/$(PKG_ID)/main:$(PKG_VERSION) \
 		--platform linux/amd64 \
 		--build-arg ARCH=x86_64 \
@@ -50,7 +61,7 @@ endif
 docker-images/aarch64.tar: Dockerfile docker_entrypoint.sh
 ifneq ($(ARCH),x86_64)
 	mkdir -p docker-images
-	docker buildx build \
+	docker buildx build --builder $(BUILDER) --progress=plain \
 		--tag start9/$(PKG_ID)/main:$(PKG_VERSION) \
 		--platform linux/arm64 \
 		--build-arg ARCH=aarch64 \
@@ -60,9 +71,17 @@ ifneq ($(ARCH),x86_64)
 endif
 
 # ── Pack ─────────────────────────────────────────────────────────────
+ifeq ($(ARCH),x86_64)
+DOCKER_DEPS := docker-images/x86_64.tar
+else ifeq ($(ARCH),aarch64)
+DOCKER_DEPS := docker-images/aarch64.tar
+else
+DOCKER_DEPS := docker-images/x86_64.tar docker-images/aarch64.tar
+endif
+
 $(PKG_ID).s9pk: manifest.yaml instructions.md icon.png LICENSE \
                 scripts/embassy.js \
-                docker-images/x86_64.tar docker-images/aarch64.tar
+                $(DOCKER_DEPS)
 ifeq ($(ARCH),x86_64)
 	@echo "start-sdk: packing x86_64 only"
 else ifeq ($(ARCH),aarch64)
@@ -76,4 +95,4 @@ clean:
 	rm -rf docker-images
 	rm -f $(PKG_ID).s9pk image.tar scripts/embassy.js
 
-.PHONY: all x86 arm verify install clean
+.PHONY: all x86 arm verify install clean setup