support splitting cask versions by arch

[TyceHerrman]

This implements the safe root-level version split case mentioned in #21692, while preserving the manual warning for nested unsupported forms. It allows casks brew bump --open-pr -casks --auto to emit arch-specific versions when a cask's livecheck returns split versions and updates bump-cask-pr to rewrite the root version stanza into on_arm / on_intel blocks while preserving existing sha256 behavior.

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[MikeMcQuaid]

Can you explain the motivation for this change (beyond just "samford said so", what problems it solved, what approaches you considered and rejected and how you have manually tested this yourself?