refactor: rename clic2025/validation → training, add clic2025 README

The CLIC challenge calls its development set "validation" and its
holdout set "test". In codec development, "validation" typically
implies a holdout set — the opposite of its intended use here.

Renamed validation → training to avoid confusion. Added
clic2025/README.md documenting the original CLIC naming, download
URLs, and the reason for renaming.

Author: lilith

README.md

@@ -57,9 +57,11 @@ git sparse-checkout add CID22 gb82-sc qoi-benchmark
 
 | Folder | Images | Size | Purpose |
 |--------|--------|------|---------|
-| `clic2025/validation/` | 32 | 103 MB | Tune and calibrate against these |
+| `clic2025/training/` | 32 | 103 MB | Tune and calibrate encoders against these |
 | `clic2025/final-test/` | 30 | 116 MB | Holdout set — final evaluation only |
 
+Note: The CLIC challenge calls these "validation" and "test" respectively. We renamed "validation" → "training" because in codec development, "validation" typically implies a holdout set, which is the opposite of the intended use. See [`clic2025/README.md`](clic2025/README.md) for the original naming and download links.
+
 - **Resolution**: ~2048px on long edge (varies from 878px to 2048px on short edge)
 - **Format**: Lossless PNG, 8-bit sRGB
 - **Source**: https://clic2025.compression.cc/
@@ -355,8 +357,9 @@ Includes: corrupt JPEG, color profile edge cases, transparency, gradients, white
 codec-corpus/
 ├── clic2025/                    # CLIC 2025 (Unsplash License)
 │   ├── LICENSE
-│   ├── validation/              # 32 high-res images
-│   └── final-test/              # 30 high-res images (holdout)
+│   ├── README.md                # Original naming & download links
+│   ├── training/                # 32 high-res images (CLIC calls this "validation")
+│   └── final-test/              # 30 high-res images (CLIC calls this "test")
 ├── CID22/                       # Cloudinary CID22 (CC BY-SA 4.0)
 │   ├── LICENSE
 │   └── CID22-512/

clic2025/README.md

@@ -0,0 +1,31 @@
+# CLIC 2025
+
+**Challenge on Learned Image Compression 2025** — High-resolution photographic images for compression quality research.
+
+## Folders
+
+| Folder | Images | Size | Purpose |
+|--------|--------|------|---------|
+| `training/` | 32 | 103 MB | Tune and calibrate encoders against these images |
+| `final-test/` | 30 | 116 MB | Holdout set — use for final evaluation only |
+
+## Original Naming
+
+The CLIC 2025 challenge uses different names for these sets. Our folder names differ from the originals to better reflect their purpose in codec development:
+
+| Our Name | Original CLIC Name | Original Filename |
+|----------|--------------------|--------------------|
+| `training/` | "Validation set" | `clic2025_image_validation.zip` |
+| `final-test/` | "Test set" | `clic2025_image_test.zip` |
+
+In the CLIC challenge context, "validation" means the set participants use during development (to tune and iterate on), and "test" means the final holdout evaluation. We renamed "validation" to "training" because in codec development, "validation" typically implies a holdout set, which is the opposite of its intended use here.
+
+## Details
+
+- **Resolution**: ~2048px on longest edge (varies)
+- **Format**: Lossless PNG, 8-bit sRGB
+- **Source**: https://clic2025.compression.cc/
+- **Downloads**:
+  - Validation (our `training/`): https://d152a8jkvz9wzs.cloudfront.net/data/clic2025_image_validation.zip
+  - Test (our `final-test/`): https://storage.googleapis.com/clic_datasets/clic2025_image_test.zip
+- **License**: [Unsplash License](https://unsplash.com/license) — Free for any use, no attribution required.