fix(climb): only engage slide when backward direction is downhill

The slide gate previously used |forwardAlongStrand.y|, so it ignored
the sign of the slide direction. On shallow ropes (chord <30 deg from
horizontal) where computeForwardIsLast picks forward from player look,
forward can resolve to downhill. Holding S + sprint then activated the
slide and sent the player rapidly uphill, which contradicts the
gravity-assisted intent of the mechanic.

Add forwardAlongStrand.y > 0 to the slide gate so the slide only
engages when "backward" along the chord actually points down. Vertical
and steeply-angled ropes are unaffected because computeForwardIsLast
already forces forward = uphill above the VERTICAL_BIAS threshold.
Holding S without sprint still works on forward-is-downhill ropes; only
the sprint turbo is suppressed.

Closes #96

Author: matejhozlar

src/main/java/dev/matejhozlar/climbableropes/ClimbController.java

@@ -381,8 +381,13 @@ private static void tickClimb(Minecraft mc, LocalPlayer player) {
         double slideDecel = ClimbableRopesConfig.SLIDE_DECELERATION.get();
 
         double verticalComponent = Math.abs(forwardAlongStrand.y);
+        // The slide is gravity-assisted descent; only engage when "backward" along the chord
+        // actually points downhill (i.e. forward points uphill). On shallow ropes where
+        // forwardAlongStrand was chosen from player look, forward can be downhill and
+        // sliding would otherwise rocket the player uphill.
         boolean slideEffective = climbDown && sprint && !descentBlocked
-                && slideSpeed * verticalComponent > descendSpeed;
+                && slideSpeed * verticalComponent > descendSpeed
+                && forwardAlongStrand.y > 0;
         if (climbUp || descentBlocked) {
             slideVelocity = 0.0;
         } else if (slideEffective) {