ci: generate Windows-on-ARM bindings on a windows-11-arm runner

[CGMossa]

TODO: replace this line with your own framing, in your own voice.

AI-written details

Summary

• Adds Windows-on-ARM (aarch64-pc-windows-gnullvm) bindings support, which extendr/libR-sys does not ship today.
• build.rs: get_r_library now handles the aarch64 arch on Windows (maps to bin/x64) instead of panicking with "Unknown architecture", so the crate can build on Windows-ARM once a bindings file exists.
• New workflow_dispatch workflow generate-windows-arm-bindings.yml: regenerates the bindings natively on a windows-11-arm runner (GA 2025-08-07, free for public repos), pins the last pre-detach generator commit for build.rs to drive bindgen, validates with the layout tests, and uploads the result as an artifact.
• Supersedes Add Windows ARM bindings #225, whose cross-compile approach predates the bindgen detach (Detach bindgen for next release #250) and relied on CI emit machinery that has since been removed.

Why a generation workflow instead of finishing #225

• bindgen was detached from the crate in Detach bindgen for next release #250, so build.rs no longer has a generate_bindings path and the old pr_generated_bindings / artifact-emit CI flow is vestigial. The cross-compile-on-x86 approach in Add Windows ARM bindings #225 can no longer produce a file in-tree.
• Running natively on a windows-11-arm runner avoids cross-sysroot wrangling entirely (the part where Add Windows ARM bindings #225 stalled), and lets the layout tests actually execute on the target rather than being skipped.

Test plan

• Run the "Generate Windows ARM bindings" workflow from the Actions tab; confirm the bindgen build and the layout-test step both pass on the windows-11-arm runner.
• Download the bindings-windows-aarch64 artifact, commit it into bindings/, and add the bindings-windows-aarch64.rs symlink (mirroring the other targets).
• Confirm a fresh cargo build for aarch64-pc-windows-gnullvm picks up the committed file via retrieve_prebuild_bindings.

Notes

• This PR has not been run in CI yet; it is the mechanism, not the generated file. Open as a draft until the workflow goes green.
• Two values need a real run to confirm: the R bin subdirectory on ARM Windows (assumed bin/x64) and the exact aarch64 Rtools toolchain path. Both are isolated and easy to adjust if the first run disagrees.
• The pinned generator commit is exposed as a workflow_dispatch input so it can be bumped without editing the workflow.

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Drafted by Claude (claude-opus-4-8). Reviewed by the author.