README promises both architectures; only x86-64 is built #5

The README promises two architectures; only x86-64 is built

README.md says the workflow "builds packages for both architectures and
publishes them to a GitHub Release". It does not — every published asset is
x86-64 only:

$ gh release view minimax-code-v3.1.4 --json assets --jq '.assets[].name'
checksums_minimax-code_3.1.4_amd64.txt
minimax-code-3.1.4.x86_64.rpm
minimax-code_3.1.4_amd64.deb

and the build refuses anything but x64:

$ scripts/lib_common.sh:160 resolve_arch() {
    case "$1" in
      x64|amd64) ... ;;
      *) die "Unknown arch '$1' (only x64 is supported)" ;;

The workflow has a single build job on ubuntu-latest — no matrix, no
arm64 anywhere.

This is a request to implement arm64 (or, at minimum, to correct the
README sentence — a fix for that is included in #4).

Why it is feasible

Everything the arm64 build needs exists upstream:

The upstream macOS arm64 DMG is published separately from the x64 one, so
resolve-version.sh would need to select *ArmMacosDownloadUrl for an arm64
build.

Suggested approach

1. Extend resolve_arch with an arm64 case:
   MMX_ELECTRON_ARCH=arm64 MMX_NPM_ARCH=arm64 MMX_DEB_ARCH=arm64 MMX_RPM_ARCH=aarch64.
2. Build on a native arm64 runner (ubuntu-24.04-arm is available on GitHub
   Actions) rather than cross-compiling node-pty: node-pty publishes no Linux
   prebuilds, so it has to be compiled anyway, and a native builder avoids a
   cross-toolchain for pty.node.
3. Add arm64 to the workflow matrix; asset names already follow from the
   arch (..._arm64.deb, ....aarch64.rpm).
4. Verify with the launch smoke test (tests/smoke-launch.sh) — an arm64
   payload that cannot dlopen a native addon still builds cleanly.

I can put up a PR for this if it is wanted — the pieces above are all
verified to exist, but the arm64 end-to-end build has not been run yet.