feat: add shared counts and revise motion acceleration

[kibi2]

Summary

This PR updates motion acceleration and deceleration behavior and adds shared counts.

Changes

• Add shared counts between paired motion keys (j/k, h/l, w/b)
• Change deceleration to holding the opposite motion key
  • e.g. holding k while holding j reduces the motion amount
• Prevent acceleration when an invalid repeat (ng_repeat) is detected
• Limit consecutive tap acceleration to 4×; a third consecutive tap resets acceleration
• Add CI tests
• Add luacov for code coverage

Notes

The acceleration/deceleration behavior is now based on the direction of the held motion key, making the behavior more consistent with Vim's motion keys.

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