Nano RP2040 Servo pulsewidth imprecision

Observed downward fluctuations of more than ~30 us when writing a 1500 us pulse to 7 different pins.
Caused a lot of random spinning as I was using the signal for ESCs.

Used the Servo.h library as provided by this core.
```
const byte motor_pins[7] {2, 3, 5, 6, 9, 10, 11};
Servo motors[7];

void setup() {
for (int i = 0; i < NUM_MOTORS; i++) {
motors[i].attach(motor_pins[i]);
motors[i].writeMicroseconds(1500);
}
}
```

Reproduce the provided Servo.h sketch on the Nano RP2040 with seven pins and measure the 1500 us outputs. Start by tracing the Servo.h implementation used by this core and its pulse-timing path. Done means the commanded pulsewidth no longer shows downward fluctuations of more than about 30 us across the attached pins.

Written by the indexing model from the issue text.