Replace focus polling with AXObserver tracking

[FuJacob]

Summary

• Replaces the timer-based focus polling path with process-scoped AXObserver notifications for focus/window changes, value/selection changes, element destruction, and app deactivation.
• Publishes raw AXObserver hit events through the focus model and flashes a short debug-overlay pulse so we can see when observer callbacks actually fire.
• Removes pollInterval wiring from the app environment and updates focus docs/comments around the event-driven model.

Validation

xcodebuild test -scheme tabby -project tabby.xcodeproj -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO
# ** TEST SUCCEEDED **
# Executed 61 tests, with 0 failures (0 unexpected)

Note: the test process still printed the existing llama.cpp/ggml Metal teardown GGML_ASSERT([rsets->data count] == 0) after XCTest reported success; xcodebuild exited 0. I did not record a live-app screen capture for the new debug pulse yet.

Linked issues

Refs #18
Refs #16

Risk / rollout notes

• This is intentionally experimental: it removes polling entirely and does not include the fallback polling path from the full ticket acceptance criteria.
• Apps that do not reliably emit Accessibility notifications may leave focus state stale until another activation or observer event occurs.
• Non-deactivation AX notifications are coalesced with a 0.005s delay before reading AX state so value/selection attributes can settle.
• Idle CPU still needs to be measured against the old polling implementation before treating this as production-ready.