Add #5215: Add onClientMouseMove event for raw mouse delta

[x6c85]

Summary

Adds a new onClientMouseMove event that fires raw mouse deltas (deltaX, deltaY) using Windows Raw Input (WM_INPUT). Unlike onClientCursorMove which only reports clamped screen coordinates, this event works regardless of cursor visibility or camera target.

Motivation

Fixes #5215. onClientCursorMove arguments don't update when the cursor is hidden and the camera isn't targeting an entity, because it relies on WM_MOUSEMOVE which gives absolute clamped coordinates. This new event uses Raw Input to capture actual mouse deltas, enabling mouse flick detection for anti-cheats and first-person camera implementations.

Test plan

1. addEventHandler("onClientMouseMove", root, function(dx, dy) outputChatBox(dx.." "..dy) end) - move mouse with cursor hidden, verify deltas are reported.
2. Show cursor with showCursor(true) - both onClientCursorMove and onClientMouseMove fire.
3. Fix camera at a position with no target entity - onClientMouseMove still reports deltas while onClientCursorMove coordinates stay clamped.
4. Verify no performance regression from Raw Input registration.

Checklist

• Your code should follow the coding guidelines.
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