feat: optimize fillRectDither (#737)

## Summary

This PR optimizes the `fillRectDither` function, making it as fast as a
normal `fillRect`

Testing code:

```cpp
{
auto start_t = millis();
renderer.fillRectDither(0, 0, renderer.getScreenWidth(), renderer.getScreenHeight(), Color::LightGray);
auto elapsed = millis() - start_t;
Serial.printf("[%lu] [ ] Test fillRectDither drawn in %lu ms\n", millis(), elapsed);
}

{
auto start_t = millis();
renderer.fillRect(0, 0, renderer.getScreenWidth(), renderer.getScreenHeight(), true);
auto elapsed = millis() - start_t;
Serial.printf("[%lu] [ ] Test fillRect drawn in %lu ms\n", millis(), elapsed);
}
```

Before:

```
[1125] [ ] Test fillRectDither drawn in 327 ms
[1347] [ ] Test fillRect drawn in 222 ms
```

After:

```
[1065] [ ] Test fillRectDither drawn in 238 ms
[1287] [ ] Test fillRect drawn in 222 ms
```

## Visual validation

Before:

<img width="415" height="216" alt="Screenshot 2026-02-07 at 01 04 19"
src="https://github.com/user-attachments/assets/5802dbba-187b-4d2b-a359-1318d3932d38"
/>

After:

<img width="420" height="191" alt="Screenshot 2026-02-07 at 01 36 30"
src="https://github.com/user-attachments/assets/3c3c8e14-3f3a-4205-be78-6ed771dcddf4"
/>

## Details

The original version is quite slow because it does quite a lot of
computations. A single pixel needs around 20 instructions just to know
if it's black or white:

<img width="1170" height="693" alt="Screenshot 2026-02-07 at 00 15 54"
src="https://github.com/user-attachments/assets/7c5a55e7-0598-4340-8b7b-17307d7921cb"
/>

With the new, templated and more light-weight approach, each pixel takes
only 3-4 instructions, the modulo operator is translated into bitwise
ops:

<img width="1175" height="682" alt="Screenshot 2026-02-07 at 01 47 51"
src="https://github.com/user-attachments/assets/4ec2cf74-6cc0-4b5b-87d5-831563ef164f"
/>

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? **NO**