fix(macos): don't panic on unexpected event types in keyboard callbacks

[paravozz]

Summary

KeyboardState::process_native_event (src/macos/keyboard.rs:282) is called from the macro-generated keyDown: / keyUp: / flagsChanged: selector implementations in src/macos/view.rs:77. The match event.eventType() in the function only covers NSKeyDown, NSKeyUp, and NSFlagsChanged, falling through to _ => unreachable!() for anything else.

Because the selector functions are generated as extern "C", hitting unreachable!() crosses the FFI boundary as panic_cannot_unwind and aborts the hosting process. For plug-ins, that crashes the DAW.

Evidence

Captured from a baseview-based plug-in running in Ableton Live 12.3.6 on macOS, 2026-04-24:

info:     panicked at library/core/src/panicking.rs:225:5:
panic in a function that cannot unwind
backtrace:
  ...
 10: core::panicking::panic_nounwind_fmt::runtime
 11: core::panicking::panic_nounwind_fmt
 12: core::panicking::panic_nounwind
 13: core::panicking::panic_cannot_unwind
 14: flagsChanged
           at src/macos/view.rs:81:9
 15: <unknown>
 16: <unknown>

(The inner panic frame that actually tripped unreachable!() was optimized out; frame 14 is the FFI boundary function. Given the match structure, the unreachable is the candidate.)

Why this hits in practice

AppKit occasionally dispatches non-key events into the key selectors during Cmd-Tab transitions, input-source switches, modifier-repeat sequences, and sleep/wake cycles. Even if the specific crash above was not unreachable!() itself, any unexpected NSEvent::eventType() value (NSAppKitDefined, NSSystemDefined, etc.) reaching this function is a latent crash.

Fix

Replace _ => unreachable!() with _ => return None. The handler silently drops events it cannot interpret. There is nothing meaningful to synthesize from an event that is not a key or flags-changed, and panicking from an extern "C" callback is never the right move.

The NSFlagsChanged arm above this already has a return None branch for the non-modifier-key subcase (line 311 on master), so the return-None fallback is consistent with existing handling of uninteresting events.

Test plan

• cargo check --features opengl green.
• Verified on a baseview-based plugin in both Ableton Live 12.3.6 and REAPER 7.69 on macOS 15.5: normal typing, backspace, enter, arrow keys, modifier combinations, Cmd-Tab in and out, input-source switch, all unchanged. No new crashes observed.

Local CI notes: the cargo fmt --check and cargo clippy -D warnings errors on master appear to be pre-existing (upstream objc 0.2.7's cargo-clippy cfg warnings and a window.rs formatting drift that I did not touch). Happy to adjust scope if you want the cfg-warning suppression bundled into this PR or handled separately.