Fix Cocoa modifier recovery and duplicate control input

[elcritch]

Summary

• derive Cocoa modifier key transitions from the current CoreGraphics per-key state instead of toggling cached state
• clear the modifier snapshot before emitting synthesized focus-loss releases
• keep Control and Command key chords out of the Cocoa text-input context so one physical chord cannot emit both key and text events
• recover correctly when an app regains focus while a modifier is already held, such as during Cmd-Tab

Regression sequences

When the app became key while Command was held, refreshModifiers restored the generic modifier mask but the pressed-key set remained empty. The following Command release was therefore toggled into the pressed set as a new press, leaving Command stuck for later key events.

Cocoa also routed Control-letter chords through both the key event callback and NSTextInputClient. Terminal consumers therefore received control bytes such as Control-B and Control-F twice.

Verification

• nim c --hints:off tests/t_multiwindow.nim
• nim c --hints:off tests/t_event_loop.nim
• downstream NimKit Bash input regression suite