ci(build): first-class cross-compilation for embedded targets #431

Background

fat.yml (added in #427, extended in #428) verifies fat builds only on native x86_64 Linux/macOS. Embedded users cross-compile; nothing today guarantees make fat-dist survives an ARM toolchain.

Proposal

• Makefile: honor CROSS_COMPILE=<triplet>- (or explicit CC/AR/RANLIB) uniformly, including the vendored cmake builds (BoringSSL/lsquic toolchain files).
• CI: aarch64-linux-gnu and arm-linux-gnueabihf cross builds of fat-dist, with the embedded-style link smoke run under qemu-user.
• Packaging: a Yocto recipe (meta-cwist layer) and a Buildroot package, plus a short "deploy to a Raspberry Pi-class board" tutorial.

Why

Cross-compiling is the embedded norm; a verified CROSS_COMPILE=... make fat-dist one-liner plus board recipes removes the main adoption friction after #427.