* ci: add linux-arm64 (aarch64) prebuilt release target
Build an omnigraph-linux-arm64 archive in both the tagged-release and
edge-release matrices on the ubuntu-24.04-arm runner, and teach the
install script to map Linux/aarch64 to the new asset. Update the install
and CI docs to list the new platform.
Previously aarch64 Linux hit the install-script arch guard
("no prebuilt binary is available for Linux/aarch64") and could only
build from source; it is now a first-class prebuilt target.
* ci: emit a linux-arm64 bottle in the Homebrew formula
The formula generator only resolved macos-arm64 and linux-x86_64 and
emitted `on_linux { on_intel }`, so `brew install` on Linux/aarch64 had
no URL/sha and failed even though the release now ships an
omnigraph-linux-arm64 archive. Resolve that asset's digest and add an
`on_arm` block under `on_linux` so the documented Homebrew path matches
the new prebuilt target.
