docs(input-latency): correct against the ash renderer and the real de…

[joschaschmiedt]

…vice set

INPUT_LATENCY.md had drifted from the code in two directions.

Input side (§4–§6, §8):

• §5 documented AnimExternalPos, AnimZmqPos, AnimMousePos and AnimGamepadPos. None exist. The only variant is ExternalPosition2D, and it is a stub whose advance arm returns false without ever reading shm. Say so.
• The sketch used impl Animation for … over Box<dyn Stimulus>; trait objects are ruled out in favour of the flat enum.
• The design assumed every source is an absolute position. A wheel or encoder is cumulative and a gamepad stick is a rate. Document all three semantics, and why a cumulative accumulator (not a per-write delta) is the only race-free contract.
• §4 recommended the shared_memory crate, never a dependency. The vtl crate already solves shm here; mirror it. Add the seqlock, f64 accumulators and the writer heartbeat that a bare f32[2] lacks — the heartbeat is a safety requirement, not a nicety.
• Shared memory is a backend, not the interface: gamepad and keyboard are in-process, which yields --input-override for free.
• Devices are declared in rig-config.toml, not in every animation.

Render side (§10–§14):

• §10 used wgpu::PresentMode and a clap CLI snippet. The renderer is ash and the server does not depend on clap. FIFO is already the implemented default.
• §11 claimed presentation timestamps must be emulated because "wgpu does not expose" them. The DRM backend already latches real vblank timestamps and FrameStats is implemented; the actual gap is that none of it is exposed over the protocol.
• §12.1 had the render thread taking the read lock first; it takes the write lock.
• §13.3/§14 documented Windows. There is no target_os = "windows" code.
• §13.4 said to measure before adding core_affinity; rig_config.rs already parses render_cpu_core and ignores it (Apply CPU core affinity from config (currently parsed but ignored) #65).

Refs #77, #78, #79, #80, #81

Claude-Session: https://claude.ai/code/session_01TovGgCFRVaGjUhKHLVKgUF