Closes #5695

Summary

Releases only ship x86_64 Linux packages, while Windows and macOS already get arm64 builds. This adds a linux-arm64 entry to the release matrix (ubuntu-22.04-arm, same Ubuntu version as the x64 release job) and to the PR build matrix (ubuntu-24.04-arm). Both are native arm64 runners, so electron-rebuild builds the native modules for arm64 without any cross-compiling.

• New build:linux:x64 and build:linux:arm64 scripts, like the existing per-arch mac and win scripts. build:linux is unchanged for local builds.
• The arm64 build makes AppImage, deb, rpm and tar.gz, named marktext-linux-arm64-<version>.<ext>. The x64 file names stay exactly the same, so existing download links keep working.
• snap is left out on arm64. electron-builder only has its prebuilt snap template for x64, and an arm64 snap would need snapcraft on the runner.
• merge-release-artifacts.py now also checks latest-linux-arm64.yml (the file the AppImage updater reads on arm64) the same way it already checks latest-linux.yml.
• RELEASE.md is updated for the 6-entry matrix and the new asset count (29).

Type of change

• Bug fix (non-breaking, fixes an issue)
• New feature (non-breaking, adds functionality)
• Breaking change (causes existing functionality to change)
• Documentation update

Test plan

• New tests added (or explain why not needed)
• Manually tested on: Linux (GitHub Actions runners, details below)

There is no test suite for the workflows or the merge script, so we tested it like this:

• Ran the PR build on our fork with only the Linux entries: https://github.com/kukirin-ui/marktext/actions/runs/38985064366. x64 on ubuntu-latest, arm64 on ubuntu-24.04-arm, and arm64 on ubuntu-22.04-arm (the release runner). All three passed. The x64 job still produces marktext-linux-0.22.0-dev.{AppImage,snap,deb,rpm,tar.gz}, and the arm64 jobs produce marktext-linux-arm64-0.22.0-dev.{AppImage,deb,rpm,tar.gz}.
• Downloaded the arm64 artifact: the deb has Architecture: arm64, and the marktext binary plus the native modules (keytar, ced, native-keymap) are aarch64 ELF files.
• A second run with dist/*.yml uploaded (https://github.com/kukirin-ui/marktext/actions/runs/38985826222) shows electron-builder writes latest-linux-arm64.yml pointing at the arm64 AppImage.
• Ran merge-release-artifacts.py on mock artifacts for all six builds. It merges cleanly, and it fails as expected when latest-linux-arm64.yml is missing or an arm64 asset is truncated.
• Smoke-tested the arm64 packages from the first PR build run above (38985064366) on the native arm64 runners ubuntu-24.04-arm and ubuntu-22.04-arm, under Xvfb: https://github.com/kukirin-ui/marktext/actions/runs/39000909285. On both, the deb installs with apt, marktext --version reports v0.22.0-dev ... OS: Linux arm64, and the installed app (with the normal Chromium sandbox) and the AppImage (FUSE, --no-sandbox) both open a sample .md file, stay running, and show a "sample.md - MarkText" window. Screenshots and logs are uploaded as run artifacts.

Notes for reviewers

We did not run the tag-triggered release workflow itself, since it publishes a release. We tested the same build steps through the PR build. The arm64 smoke test ran on GitHub Actions arm64 runners with a virtual display (Xvfb), not on a physical arm64 machine or a desktop session. The reporter offered to test it on Kylin and Ubuntu arm64 hardware.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.