Add corpus benchmarks and lattice refinement

examples/corpus/README.md

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+# Corpus Layout
+
+Put Creative Commons source pixel art in [originals](</E:/Projects/repixelizer/examples/corpus/originals>).
+If you import sprite sheets or collage sheets first, run `repixelize prepare-corpus --corpus-dir examples/corpus` to archive the raw sheets into `source-sheets/` and rewrite `originals/` into one-sprite-per-file inputs for benchmarking.
+That step also refreshes [ATTRIBUTION.md](</E:/Projects/repixelizer/examples/corpus/ATTRIBUTION.md>) from the processed sidecars.
+
+Recommended layout:
+
+- `examples/corpus/originals/<name>.png`
+- `examples/corpus/originals/<name>.json`
+
+The JSON sidecar is optional, but useful for attribution and notes. Suggested fields:
+
+```json
+{
+  "title": "Sprite Name",
+  "author": "Artist Name",
+  "license": "CC-BY 4.0",
+  "source_url": "https://example.com/original",
+  "notes": "Strong outline, metallic highlights."
+}
+```
+
+Run the round-trip benchmark with:
+
+```powershell
+repixelize prepare-corpus --corpus-dir examples/corpus
+repixelize benchmark --corpus-dir examples/corpus --out-dir artifacts/benchmark
+```
+
+By default the benchmark locks the target size to the original image dimensions so we can measure recovery quality cleanly. Add `--infer-size` if you want to evaluate automatic size inference too.

examples/corpus/originals/.gitkeep

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+