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/FirstPersonMode.cs

@@ -1,8 +1,5 @@
 using UnityEngine;
 using Unity.Netcode;
-#if ENABLE_INPUT_SYSTEM
-using UnityEngine.InputSystem;
-#endif
 
 /// <summary>
 /// First-person mode. The player keeps walking (WASD, camera-relative) but the camera moves to the
@@ -64,7 +61,7 @@ public class FirstPersonMode : GameMode
     {
         if (!CanShoot()) return;
         if (Time.time < _nextShootTime) return;
-        if (!ClickedThisFrame()) return;
+        if (!InputUtil.LeftClickedThisFrame()) return;
 
         _nextShootTime = Time.time + Mathf.Max(0f, MatchManager.Instance.ShootCooldown);
         TryShoot();
@@ -107,15 +104,6 @@ public class FirstPersonMode : GameMode
         }
     }
 
-    private static bool ClickedThisFrame()
-    {
-#if ENABLE_INPUT_SYSTEM
-        return Mouse.current != null && Mouse.current.leftButton.wasPressedThisFrame;
-#else
-        return Input.GetMouseButtonDown(0);
-#endif
-    }
-
     private sealed class RaycastDistanceComparer : System.Collections.Generic.IComparer<RaycastHit>
     {
         public static readonly RaycastDistanceComparer Instance = new RaycastDistanceComparer();