feat(presentation): raise frame-scheduler thread priority

[SpaceK33z]

Summary

Elevates the laser-frame-scheduler thread to ThreadPriority::Max so its pacing sleeps are less likely to be preempted under system load, reducing output timing jitter. Resolves the follow-up deferred from #26.

The call is best-effort: on systems that disallow raising priority without privileges (e.g. Linux without CAP_SYS_NICE) it logs a warning and continues at default priority rather than failing the session.

Why Max and not true realtime

thread-priority's set_current_thread_priority maps the requested priority into the current scheduling policy's allowed range rather than forcing a realtime policy. So Max:

• needs no elevated privileges on macOS/Windows,
• falls back to niceness under the default SCHED_OTHER on Linux,

avoiding the system-starvation and privilege pitfalls of SCHED_FIFO while still nudging the scheduler in our favor.

Changes

• Cargo.toml / Cargo.lock: add cross-platform thread-priority = "3" (always-on).
• src/presentation/session.rs: raise priority as the scheduler thread's first action (inside the closure, so it targets the correct OS thread); warn-and-continue on failure.

Testing

• cargo build clean, cargo clippy clean.
• Presentation test suite passes (96 tests).
• Manual smoke test confirmed the priority call returns Ok on macOS without special privileges.

Closes #35