Fix slow Control + arrow desktop switching

[eriko13]

Fixes #15

What this does

Control + Left/Right now switches Spaces instantly, like a three-finger swipe. Before, it used macOS's slow slide animation.

• On by default. Toggle in the menu, or run strafe control-arrows off.
• Uses the Accessibility permission strafe already has.
• Keep macOS's "Move left/right a space" shortcuts enabled in System Settings.
• Replaces the old Control+Option+arrow shortcuts and the strafe hotkeys command. Use strafe control-arrows [on|off] instead.

How it works

1. While you hold Control, strafe pauses macOS's two Space shortcuts in memory.
2. A hotkey on Control+Left/Right runs strafe's instant switch.
3. When you release Control, the macOS shortcuts come back.

The pause is needed because macOS handles Control+arrow before any app sees it, so a hotkey alone never fires (tested).

What strafe can see

• The Control watcher only receives modifier changes. Letters, numbers and arrow keys never reach strafe. Nothing is stored or logged.
• The hotkey fires only for the exact Control+Left/Right combo.
• System Settings and com.apple.symbolichotkeys are never edited.

Trade-offs

• Uses the private WindowServer call CGSSetSymbolicHotKeyEnabled.
• If strafe crashes or is force-quit while Control is held, the two macOS shortcuts stay off until strafe relaunches (it repairs this at launch) or you log out.
• A held arrow key does not repeat the switch. Skipped while Mission Control is open.

Tested

• 13 unit tests plus the hotkey test scripts pass.
• Manually: instant in both directions, swipes still work, typing unaffected.
• Not tested: Fn/Globe combos, external keyboards, Accessibility revoked mid-session.

Changed since the earlier revision

The first version tapped all key-down/up events. Now it is modifier-only, so strafe receives no keystrokes. README and SECURITY.md updated to match.