perf: move image generation off the HA event loop (fixes #17)

[Copilot]

Summary

Fixes the HA event-loop blocking described in OpenDisplay/Home_Assistant_Integration#17.

All PIL drawing operations (text, shapes, QR codes, image downloads/transforms, icon rendering, JPEG encoding) previously ran synchronously on the HA event loop, causing HA to lock up for tens of seconds per image update.

Root causes fixed

Problem	Fix
All PIL CPU work blocked the event loop	Entire drawing pipeline moved to hass.async_add_executor_job
MDI icon JSON (~7 MB, 7 334 icons) re-read from disk on every icon element	Module-level _mdi_metadata_cache + _mdi_font_cache — loaded once, reused forever
QR-code generation (qr.make()) ran on event loop	Now in executor thread
Image resize/rotate/composite ran on event loop	Now in executor thread
requests.get() was wrapped in extra async_add_executor_job inside an already-sync handler	Simplified to a direct call (already in thread)

How it works now

Phase 1 – async (event loop)
_async_prefetch_resources() fetches only data that requires the HA event loop:

• Recorder history for plot elements (via prefetch_plot_data)

Phase 2 – sync (executor thread)
_generate_sync() runs in hass.async_add_executor_job and handles:

• PIL image creation, all drawing handlers, rotation, RGB conversion, JPEG encoding
• Blocking HTTP/file I/O in draw_downloaded_image (safe in thread)
• MDI font + metadata lookup from module-level cache

Changes

• core.py: Two-phase generate_custom_image; new _async_prefetch_resources + _generate_sync methods
• registry.py: Handler wrapper changed from async def to def
• text.py, shapes.py, debug.py: async def → def (no I/O)
• icons.py: Module-level MDI cache helpers; async def → def
• media.py: async def → def; direct blocking I/O (in thread)
• visualizations.py: New async prefetch_plot_data; all handlers async def → def
• types.py: DrawingContext.resources field added (with docstring)
• tests/drawcustom/conftest.py: async_add_executor_job mock now executes callables so existing 82 tests still pass