fix: preserve modifier releases from Windows to macOS

[Jayocean2000]

When a Windows host shares input with macOS, ordinary clicks can intermittently behave as Ctrl-clicks after the physical modifier has been released. This change addresses two paths that can leave the remote key state inconsistent.

• Windows capture uses a non-blocking unbounded channel instead of a capacity-10 channel whose try_send can discard key/button releases behind mouse motion. Hook callbacks also pass negative hook codes to the next hook before reading event data.
• macOS posts each received key press/release independently of the repeat task. Only non-modifier keys repeat, cancelling repeat no longer synthesizes a key release, and releasing another key does not cancel the currently repeating key. Destroy/terminate stop the repeat task. Pointer events explicitly use the received modifier state.

Validation:

• cargo fmt --all --check and git diff --check passed.
• cargo test --workspace passed on macOS, including three modifier/repeat regression tests.
• cargo clippy --workspace --all-targets --all-features -- -D warnings passed on macOS.
• The unchanged Windows capture changes were built and tested natively on Windows with cargo test -p input-capture -p input-emulation --no-default-features; the motion-burst/release regression passed. A Windows GTK release build was deployed for initial Windows-to-macOS use.
• Initial manual use reports that the intermittent Ctrl-click symptom appears resolved; prolonged stress testing is still needed. The published macOS source preserves the latest upstream key mapping; the deployed test build additionally contains local click-focus changes that are excluded from this PR.

Known limitations:

• Win+G still opens Xbox Game Bar on the Windows host while input is captured for macOS. This PR does not fix that shortcut behavior. The README records this observed limitation.
• An unbounded Windows queue can grow if the consumer stalls. This avoids losing release events or blocking a low-level hook; motion coalescing may be needed if sustained backlog is observed.

No protocol or configuration changes.