Red koopa shell: fix circling, add terrain step-up, clean homing algorithm

[Copilot]

Red koopa shells circled targets rather than hitting them, and were useless on any non-flat terrain.

Changes

Fix circling near targets

The P-controller gain was too low (0.1), causing the shell to take ~10 ticks to align — enough to orbit close targets indefinitely. Raised to 0.25 via a named constant HOMING_TURN_RATE.

// before: hard-coded, too slow to turn
double pGain = 0.1;

// after: fast enough to converge before overshooting
private static final double HOMING_TURN_RATE = 0.25;

Terrain step-up

Overrides maxUpStep() returning 1.0f so the shell uses Minecraft's built-in step-up physics to climb single blocks in its path — consistent with how players and mobs navigate terrain.

Homing algorithm cleanup

• Extracted currentVelocity.multiply(1, 0, 1) into currentHorizontal (was duplicated)
• length() → lengthSqr() (avoids unnecessary sqrt)
• Replaced == 0 float equality with < 1e-10 epsilon guard

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