refactor: improve Makefile reliability

- Use .unpacked stamp files for source dependencies instead of directories
 to prevent unnecessary rebuilds when builds modify source trees
- Hardcode checksums in Makefile instead of downloading from same server as tarballs
- Use Docker for all Linux builds (both on macOS and Linux) for reproducibility
- Fix gawk/grep lipo race condition by depending on directory targets with recipes
- Fix toybox parallel build race by clearing MAKEFLAGS before sub-make
- Fix clean_dist to fully remove directory instead of just contents
- Add component/platform validity matrix to prevent invalid target generation
- Simplify UNIVERSAL_DARWIN_COMPONENTS to use COMMON_COMPONENTS directly
- Add docker_platform helper for consistent Docker platform naming
- Update README with Docker requirements and OrbStack as alternative
- Disable busybox CONFIG_TC and SHA hardware acceleration on aarch64
- Add macOS SDK 26.2
- Bump versions

Author: deciduously

Dockerfile

@@ -0,0 +1,6 @@
+FROM alpine:3.23.2
+RUN apk update && apk add \
+    alpine-sdk autoconf automake bash binutils bison build-base \
+    file flex gawk gcc gcompat gettext-tiny git grep help2man \
+    indent m4 libbz2 libgcc libtool linux-headers ncurses \
+    ncurses-dev openssl-dev python3 wget xz zlib-dev zlib-static