Add opt-in realtime-like thread priority for the pacer and audio threads

[LinuxJedi]

Summary

Adds a best-effort, off-by-default scheduling-priority feature for Copperline's two latency-critical host threads — the wall-clock pacer (the main thread) and the cpal audio callback. On a busy host the scheduler can preempt either at the wrong moment, showing up as frame stutter or audio underruns; raising their priority reduces both. It never changes emulated behaviour — like all pacing, it only schedules host work.

Per-platform handling

"Real-time priority" is portable in neither API nor semantics, so the new src/priority.rs handles each platform on its own terms:

• macOS — the pacer thread joins the USER_INTERACTIVE QoS class (pthread_set_qos_class_self_np), the idiomatic unprivileged low-latency request. The audio callback is left alone: Core Audio already runs it on a real-time thread, and pinning a QoS class onto it would only demote it. Uses the libc QoS API already in the tree, so no new macOS build deps.
• Windows — both threads are raised to THREAD_PRIORITY_HIGHEST via the thread-priority crate; no privilege required.
• Linux/other Unix — raising priority needs privilege (an rtprio rlimit, CAP_SYS_NICE, or root); without it the request is logged and declined, and the thread keeps normal scheduling.

Dependency notes

• thread-priority is scoped to cfg(not(target_os = "macos")), so it (and the windows-sys it pulls in) never enters the macOS build.
• audio_thread_priority was deliberately avoided — it drags libdbus into Linux builds for RtKit, which would require packaging/CI changes.

Configuration

Gated by [emulation] realtime_priority (default false), overridable for one run by the COPPERLINE_REALTIME_PRIORITY env var (0/false/off forces off; anything else forces on). The pacer is elevated only when actually pacing (skipped for unthrottled headless captures); the audio thread promotes itself once on its first callback (latched, so steady-state audio does no extra work).

Docs

• docs/guide/configuration.md — the [emulation] reference.
• docs/internals/timing.md — a new Thread scheduling priority subsection under Real-time pacing.
• docs/internals/architecture.md — the threading model and the priority.rs source-layout entry.
• copperline.example.toml — the commented key.

Test plan

• cargo fmt --check, cargo clippy --all-targets --all-features --locked -- -D warnings, cargo build --locked — all clean (the exact CI commands).
• Full suite passes (1004 tests, including 2 new ones in priority.rs that exercise the real macOS QoS syscall and assert the wrappers are panic-free on every platform).
• Live smoke test on macOS confirmed the audio path logs audio thread left as-is (Core Audio runs it real-time) and makes the correct call.
• ⚠️ The interactive pacer path (QoS elevation in a windowed run) was not driven end-to-end locally — it needs a real window — but it compiles, is wired in main(), and uses the same syscall the unit test exercises directly.