test/chore: compare snapshots by pixels; small resource and robustness fixes

[balloob]

Summary

Fixes the brittle visual-snapshot comparison (which fails on main under Pillow 12) plus a handful of small resource/robustness notes.

Findings addressed

• C1 — visual snapshots compared raw PNG bytes. Every tests/visual/* test asserted on encoded PNG bytes, so a Pillow/zlib re-encode of pixel-identical output broke the snapshot — 29 of them fail on main under Pillow 12. Added PixelPNGImageSnapshotExtension, which decodes both sides and diffs pixels across all channels (getbbox(alpha_only=False) — the default alpha_only=True would ignore RGB changes on fully-opaque e-paper renders). The 12 text-bearing baselines are refreshed to the supported Pillow 12 rasterization (they were generated on an older Pillow and differed by ~1px, flipping some black/white text pixels); the 17 shape/QR baselines were already pixel-identical and are byte-unchanged. The suite now runs 409 passed / 0 failed (was 29 failed).
• A12 — small resource/API notes:
  • media_loader calls Image.load() for file and HTTP sources, so the file handle is released (no ResourceWarning under -W error) and decoding happens up front instead of lazily mid-composite.
  • warmup now continues past a font size that fails to load (and logs it) instead of breaking and silently skipping the rest.
  • the plot normalizes naive last_changed timestamps to UTC, avoiding a TypeError from aware-vs-naive subtraction at render time; the DataProvider protocol docstring documents the expectation.
• B9 — session-less HTTP. load_image now warns once (not every frame) when it has to create a throwaway aiohttp.ClientSession, nudging the HA integration to pass its shared session for connection pooling / DNS caching.

Tests

Adds unit tests for the pixel snapshot matcher (same pixels / different bytes → match; different pixels or size → no match; non-image → byte fallback), warmup continuing past a failing size, the file-handle release on file load, and the naive-timestamp plot path.

Note

The naive-timestamp normalization touches elements/visualizations.py, which the separate plot-correctness PR also touches (different lines — no conflict expected).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/odl_renderer/media_loader.py	40.00%	6 Missing ⚠️

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