Add SSD1333 (176×176) OLED display support + finer low-end brightness control

[brickbots]

Summary

Adds support for the SSD1333 176×176 RGB OLED display controller as a new display option, and refines low-end brightness control for both the SSD1333 and the existing SSD1351.

What's in here

New PiFinder/ssd1333_device.py — a luma.oled driver for the Solomon Systech SSD1333 (176×176, 16-bit 5-6-5 color), subclassing luma.oled.device.color_device. Notable differences from the SSD1351 are documented inline (176×176 resolution, MUX ratio 175, no GPIO/Function-Select/2nd-unlock commands, built-in linear LUT). Exposes contrast() (per-channel, 0xC1) and master_brightness() (global current control, 0xC7).

PiFinder/displays.py

• New DisplaySSD1333 display class (SPI, 176×176, BGR), wired into get_display() under the "ssd1333" hardware key.
• Reworked set_brightness() for both SSD1351 and SSD1333 to combine master brightness (0xC7) with per-channel contrast (0xC1) for a wider dimming range:
  • Levels 0–15: master and contrast scale together → dimmer than contrast alone can reach (better for night vision).
  • Levels 16–255: master at full, contrast varies linearly.

The change is additive — existing displays (SSD1351, ST7789) are unaffected aside from the SSD1351 brightness curve improvement.