feat(steami_screen): Add GC9A01 display wrapper.

[nedseb]

Description

Add a GC9A01 display wrapper for steami_screen, converting RGB tuples to RGB565 internally.

Context

The SSD1327 wrapper was added in PR #355. The GC9A01 wrapper was deferred because it cannot be tested on available hardware (Rev A has SSD1327 only, GC9A01 is on Rev B).

A draft implementation exists in Charly's branch (feat/migrate-steami-screen, commit a6074d9) with the import/docstring cleaned up.

Tasks

• Create lib/steami_screen/steami_screen/gc9a01.py — GC9A01Display wrapper converting RGB to RGB565
• Add fill_rect/rect with hasattr guard (same pattern as SSD1327Display)
• Export from __init__.py
• Add mock tests
• Test on hardware when Rev B board is available
• Update README

Reference

• SSD1327Display wrapper: lib/steami_screen/steami_screen/ssd1327.py
• Color conversion: rgb_to_rgb565() in colors.py