Add arm64 Docker build container for native cross-compilation

Uses ubuntu:22.04 arm64 image running under QEMU emulation to produce
aarch64 binaries that match the target device exactly (same glibc, same
distro). Replaces the broken Buildroot cross-compilation approach.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-arm64/Dockerfile

@@ -0,0 +1,8 @@
+FROM ubuntu:22.04
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential pkg-config \
+    libsdl2-dev libsdl2-image-dev \
+    libavformat-dev libavcodec-dev libavutil-dev libswresample-dev \
+    && rm -rf /var/lib/apt/lists/*
+WORKDIR /workspace

docker-arm64/Makefile

@@ -0,0 +1,25 @@
+.PHONY: shell build clean
+
+IMAGE_NAME = arm64-dev
+PROJECT_DIR := $(shell cd .. && pwd)
+
+CONTAINER_NAME = $(shell docker ps -f "ancestor=$(IMAGE_NAME)" --format "{{.Names}}")
+
+.build: Dockerfile
+	docker build --platform linux/arm64 -t $(IMAGE_NAME) .
+	touch .build
+
+ifeq ($(CONTAINER_NAME),)
+shell: .build
+	docker run --platform linux/arm64 -it --rm -v "$(PROJECT_DIR)":/workspace $(IMAGE_NAME) /bin/bash
+else
+shell:
+	docker exec -it $(CONTAINER_NAME) /bin/bash
+endif
+
+build: .build
+	docker run --platform linux/arm64 --rm -v "$(PROJECT_DIR)":/workspace $(IMAGE_NAME) make
+
+clean:
+	docker rmi $(IMAGE_NAME)
+	rm -f .build