feat(macos): add opt-in physical-only key events

[LuisJann]

Summary

Recent macOS versions can abort a process when TIS/TSM input-source operations
run from an inappropriate queue. An rdev listen() callback can encounter this
while resolving layout-dependent key names.

This change adds the opt-in macos_physical_key_events feature. When enabled,
macOS key events are mapped exclusively from their physical keycodes and
Event::name is left unset. The default behavior remains unchanged.

Motivation

Consumers such as keyboard-sound applications do not need Unicode or current
keyboard-layout resolution; they need stable physical key identities. For those
applications, dispatching TIS work to the main queue adds latency and potential
re-entrancy/deadlock risk, while moving the complete event tap to the
application's main run loop changes callback scheduling.

An explicit physical-only mode avoids TIS entirely without changing callback
threading or affecting existing consumers. This complements the main-thread
initialization work in #147 and is
motivated by the crash class reported in
#146.

Changes

• add the macos_physical_key_events Cargo feature;
• map key down/up events directly with key_from_code when enabled;
• avoid TIS-backed name lookup in both key classification and Event::name;
• preserve the existing conversion path when the feature is disabled;
• suppress feature-specific dead-code warnings in the unused TIS helpers.

Behavioral impact

With macos_physical_key_events enabled:

• physical key press/release and modifier events remain available;
• mouse events are unchanged;
• Event::name is None for macOS key events;
• layout-dependent fallback recognition for otherwise unknown extended keys is
  unavailable.

Without the feature, behavior and API output are unchanged.

Validation

• cargo fmt --all --check: passed;
• cargo test --lib: 4 passed with the default configuration;
• cargo test --lib --features macos_physical_key_events: 4 passed;
• an equivalent physical-only rdev 0.5.3 path was validated in MechvibesDX on
  Apple Silicon/macOS with global keyboard and mouse input, both audio paths,
  more than five minutes of stability, and close/reopen;
• no SIGTRAP, dispatch_assert_queue, or
  TSMGetInputSourceProperty crash recurred in that consumer validation.

Compatibility

The feature is opt-in and does not alter the default configuration. A consumer
can enable it while retaining other features such as unstable_grab.