Compute the timer compare register from a duty %.
For a timer with auto-reload register `ARR` (period in ticks), the compare register `CCR` controls duty cycle:
```
CCR = (ARR + 1) * duty_percent / 100
```
Implement `uint32_t pwm_ccr(uint32_t arr, uint32_t duty);` (duty 0..100, clamp values > 100).
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