Merges PR #165 and PR #172.
PR #165: The CDP spec marks canAccessOpener as a required boolean on
TargetInfo but Obscura was omitting it. Strict Rust clients like
chromiumoxide reject the event entirely and panic with "Created target
not present" when the field is absent. Added canAccessOpener: false
to every TargetInfo emitted by setDiscoverTargets, getTargets,
createTarget, attachToBrowserTarget, attachToTarget, and getTargetInfo.
PR #172: Added a cross-compile matrix entry for aarch64-unknown-linux-gnu
running on ubuntu-22.04 x86_64, producing obscura-aarch64-linux-cross.
This is a fallback for when GitHub ARM runners are unavailable without
removing the native ARM build.
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