test: add coverage for clampHorizontalSpeed and projectDirectionOnPlane

[yassin-openclaw]

Summary

Adds test coverage for two previously untested pure utility functions in `MovementMath.ts`: `clampHorizontalSpeed` and `projectDirectionOnPlane`.

Changes

• `src/movement/tests/MovementMath.test.ts` — added 7 new test cases across two new `describe` blocks

clampHorizontalSpeed (4 cases):

• Speed already under limit → no modification
• Speed over limit → clamped to max
• Direction preserved after clamping
• Zero vector → no division by zero

projectDirectionOnPlane (3 cases):

• Normal case → result is unit length and perpendicular to plane normal
• Direction parallel to normal → returns zero vector
• Input vectors not mutated

Validation

```
npm test
✓ MovementMath.test.ts (10 tests)
✓ MovementBehavior.test.ts (3 tests)
✓ SurfCollisionWorld.test.ts (3 tests)
Tests: 16 passed (16)
```

Risks / Notes

• Pure unit tests only; no production code changed
• No new dependencies introduced