ci: add linux-arm64 (aarch64) prebuilt release target (#316)

* 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.

.github/workflows/release-edge.yml

@@ -41,6 +41,8 @@ jobs:
         include:
           - runner: ubuntu-latest
             asset_name: omnigraph-linux-x86_64
+          - runner: ubuntu-24.04-arm
+            asset_name: omnigraph-linux-arm64
           - runner: macos-14
             asset_name: omnigraph-macos-arm64
           - runner: windows-latest

.github/workflows/release.yml

@@ -35,6 +35,8 @@ jobs:
         include:
           - runner: ubuntu-latest
             asset_name: omnigraph-linux-x86_64
+          - runner: ubuntu-24.04-arm
+            asset_name: omnigraph-linux-arm64
           - runner: macos-14
             asset_name: omnigraph-macos-arm64
           - runner: windows-latest