[zibo-chen]

Change

Desktop peers upgraded to clipboard replication v3 authenticate successfully, then disconnect because the welcome check only accepts v1 and v2. Validate the selected version against the same protocol constant advertised in the hello, while retaining the legacy control path for older negotiated versions and rejecting missing, unselected, zero, and future versions.

Add a real paired QUIC handshake regression through the desktop connection manager. It checks that the current version reaches the connected state, registers the replica transport, and transfers distinct text records in both directions without closing the control connection.

The integration packaging job also failed on Xcode 27 because the share extension inherited a macOS 10.13 target. Set that extension's minimum to macOS 12.0 in both the XcodeGen source and checked-in project so the CI package can be produced.

Validation

• All 14 desktop clipboard sync tests pass.
• Restoring the old version guard makes both the current-version validation and real handshake regressions fail.
• cargo fmt --all -- --check
• cargo check --workspace --all-targets
• cargo clippy -p arcrelay-desktop --all-targets -- -D warnings
• Share extension compiled with Xcode 27 for arm64 and x86_64; verified the resulting binary and macOS 12.0 minimum.

Public boundary

• This change contains no proprietary mobile source, credentials, internal URLs, product design sources, QA capture images, or internal test reports.
• I have completed the ArcRelay Contributor License Agreement, or I am contributing on behalf of the project owner.

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