(release/25.2) darwin: Set thread priorities to user interactive or user initiated as appropriate

[metux]

Backport of #3557 (master)

Signed-off-by: Jeremy Huddleston Sequoia jeremyhu@apple.com

[metux]

🤖 Automated review — generated by Starfleet ship Discovery on behalf of @metux. Not a human review.

Clean Darwin-only QoS improvement — no objections.

What it does: sets macOS thread QoS classes via pthread_attr_set_qos_class_np at thread creation time. Three XQuartz threads (X11Application.m, darwinEvents.c, quartzStartup.c) plus the cross-platform input thread (os/inputthread.c, guarded by #ifdef __APPLE__). QOS_CLASS_USER_INTERACTIVE for the startup and input threads (latency-critical), QOS_CLASS_USER_INITIATED for the two worker threads.

Runtime availability: each call is guarded by &pthread_attr_set_qos_class_np — standard weak-symbol pattern for safe fallback on older macOS. No link-time dependency added.

ABI impact: none. Darwin-only code, no struct or export changes on Linux. NVIDIA blobs unaffected.

Backport candidate: no. Performance/scheduling improvement on Darwin, not a correctness or security fix.

Verdict: passes review.