fix: project velocity and acceleration onto 2D axes in stepAlongPlane

[TechGenius-Karan]

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🔍 Description

stepAlongPlane() accepted angleRad as a third argument from its caller but silently discarded it. The 2D state.velocity and state.acceleration were synced as purely horizontal vectors (v, 0) regardless of the plane angle, which is incorrect.

Fixed by accepting angleRad (defaulting to 0) and projecting the scalar plane values onto 2D axes using cos/sin, so state.velocity and state.acceleration correctly reflect the actual direction of motion along the inclined surface.

Closes #BUG-004 (if applicable)

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🧩 Additional Notes for Reviewers

[mattqdev]

Hey @TechGenius-Karan can you please explain better (maybe with a clip of the bug or idk) whats the issue you fixed

[TechGenius-Karan]

Happy to clarify! Here's a concrete example of what was wrong and what the fix does.

The problem in one sentence: The velocity and acceleration stored in state.velocity / state.acceleration always
pointed purely horizontal — no matter what angle the plane was set to.

Concrete example — 30° incline, ball moving at 5 m/s along the plane:

Before fix:
state.velocity = (5, 0) ← always horizontal, ignores the 30° angle

After fix:
state.velocity = (5·cos30°, 5·sin30°)
= (4.33, 2.5) ← correctly along the inclined direction

Visually:

Before (wrong): After (correct):
↗ plane ↗ plane
→ velocity arrow ↗ velocity arrow
(points flat) (follows the slope)

Why it matters: state.velocity is what gets read when drawing velocity/acceleration vector arrows on screen, and
when other parts of the code ask "how fast is the body moving and in what direction?" With the old code, those
vectors always pointed flat-right regardless of slope — visually wrong and giving incorrect component values to
anything downstream that used them.

The fix (3 lines in stepAlongPlane()):
// Before:
this.state.velocity.set(v, 0);

// After:
this.state.velocity.set(v * Math.cos(angleRad), v * Math.sin(angleRad));
this.state.acceleration.set(a * Math.cos(angleRad), a * Math.sin(angleRad));

Hope this clarifies your doubt better.
@mattqdev

[mattqdev]

Oh yeah all clear now! Was an easy one to understand, I'm sorry I'm so tired I feel dumb

[mattqdev]

Anyway thank you for this!