Fix cylinder wall-climb strafe lock and add terrain pipette support

- PlayerCharacterController: latch the wall-strafe sign once at strafe
  start instead of re-deriving it from the camera every frame, which
  flipped at the 90-degree point while circling a cylinder and caused a
  back-and-forth lock. Flat walls stay bit-identical; reset on enter/exit.
- MeshPainter.Pick: dispatch terrain hits to a splat-layer blend (full
  PBR channels), terrain trees / grass / sky to rendered screen-color,
  keeping texel-accurate picks for the paint mesh and foreign materials.
- Includes Unity auto-reserialization of SampleScene and three Supercyan
  pack assets (URP material upgrades, skybox/lighting) from the import.

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

Assets/Scripts/PlayerCharacterController.cs

@@ -160,6 +160,7 @@ public class PlayerCharacterController : MonoBehaviour
     private float _lostContactTimer;
     private float _wallOffset;            // current lean-out distance from the wall (mouse drag)
     private float _wallYaw;               // body yaw offset from facing the wall (mouse drag); held until leaving
+    private int _wallStrafeSign;          // latched sideways direction (camera picks it once per strafe); 0 = unlatched
     private bool _offsetDragging;         // true while the offset/rotation is being set (pauses camera orbit)
     private bool _climbLeftGround;        // true once the capsule has left the ground while climbing (enables ground-detach)
     private readonly RaycastHit[] _wallProbe = new RaycastHit[8];
@@ -365,6 +366,7 @@ public class PlayerCharacterController : MonoBehaviour
             _slideSpeed = 0f;
             _wallOffset = 0f;
             _wallYaw = 0f;
+            _wallStrafeSign = 0;
             _climbLeftGround = false;
         }
     }
@@ -447,8 +449,21 @@ public class PlayerCharacterController : MonoBehaviour
             if (wallUp.sqrMagnitude > 1e-6f) wallUp.Normalize();
             Vector3 wallRight = Vector3.Cross(_wallNormal, wallUp);
             if (wallRight.sqrMagnitude > 1e-6f) wallRight.Normalize();
-            if (cameraTransform && Vector3.Dot(cameraTransform.right, wallRight) < 0f)
+
+            // Pick the strafe sign ONCE from the camera when a strafe begins, then hold it.
+            // Re-deciding it every frame flips wallRight while rounding a curved (cylindrical)
+            // wall: exactly at 90 deg to the camera the dot crosses zero, so the sideways input
+            // inverts frame-to-frame and you lock there, jittering back and forth. Latching keeps
+            // a strafe continuous around the curve; it resets when the strafe input releases so
+            // the next one re-syncs to the camera.
+            bool strafing = Mathf.Abs(h) > 1e-4f;
+            if (!strafing)
+                _wallStrafeSign = 0;
+            else if (_wallStrafeSign == 0)
+                _wallStrafeSign = (cameraTransform && Vector3.Dot(cameraTransform.right, wallRight) < 0f) ? -1 : 1;
+            if (_wallStrafeSign < 0)
                 wallRight = -wallRight;
+
             alongSurface = wallUp * v + wallRight * h;
         }
         if (alongSurface.sqrMagnitude > 1f) alongSurface.Normalize();
@@ -760,6 +775,7 @@ public class PlayerCharacterController : MonoBehaviour
         _verticalVelocity = 0f;
         _wallOffset = 0f;
         _wallYaw = 0f;
+        _wallStrafeSign = 0;
         _offsetDragging = false;
         _climbLeftGround = false;
         _mantling = false;