Adopt pad_dpad_x / pad_dpad_y in directional samples

[jonathanpeppers]

The 0.3.1-beta release added pad_dpad_x() and pad_dpad_y() (#462) which return signed -1/0/+1 deltas, collapsing 4-branch directional code into 2 simple increments.

Samples to update

• snake
• pong
• climber
• metacursor
• movingsprite
• game2048
• siegegame
• shoot2

Acceptance criteria

• 4-way PAD.LEFT/RIGHT/UP/DOWN branches that adjust coordinates replaced with x += pad_dpad_x(0) / y += pad_dpad_y(0) style code where the semantics match.
• Gameplay behavior unchanged.
• .verified.bin updated if ROM bytes change.
• All tests pass.