Refactor: dedupe input reads, material-property guards, button hooks

Extract byte-identical duplicated code into shared helpers (behaviour-neutral):
- New InputUtil.cs: single source for mouse-delta/scroll/cursor-lock, modifier
  and mouse-button reads, previously copied verbatim across camera controllers,
  MeshPainter, FirstPersonMode and PlayerCharacterController.
- GetMatFloat/SetMatFloat replace the 12x material-instance + HasProperty guard
  pattern for the camo/height float properties.
- Route two inline button hooks through the existing Hook() helper.

Bespoke variants deliberately kept local (FreeCam SprintHeld = LeftShift only,
MeshPainter.TryGetMouse with out-params).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/PaintHud.cs

@@ -455,8 +455,7 @@ public partial class PaintHud : MonoBehaviour
             if (btn)
             {
                 int idx = i; // capture per swatch
-                btn.onClick.RemoveAllListeners();
-                btn.onClick.AddListener(() => SelectMask(idx));
+                Hook(btn, () => SelectMask(idx));
             }
         }