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/MeshPainter.Setup.cs

@@ -152,8 +152,8 @@ public partial class MeshPainter
         // A = smoothness. G = 128 (= 0.5) is the NEUTRAL height: no parallax/relief and AO =
         // saturate(2*0.5) = 1 (no darkening), so the unpainted look is unchanged. Painting G
         // below 0.5 carves a hole (and darkens), above 0.5 raises a hill (see PlayerCamoLit).
-        float metallic0 = _materialInstance.HasProperty(MetallicId) ? _materialInstance.GetFloat(MetallicId) : 0f;
-        float smoothness0 = _materialInstance.HasProperty(SmoothnessId) ? _materialInstance.GetFloat(SmoothnessId) : 0.5f;
+        float metallic0 = GetMatFloat(MetallicId, 0f);
+        float smoothness0 = GetMatFloat(SmoothnessId, 0.5f);
         _seedCanonical[MAP_MSO] = FillCanonical(new Color32(
             (byte)(Mathf.Clamp01(metallic0) * 255f), 128, 0, (byte)(Mathf.Clamp01(smoothness0) * 255f)));