docs: add README with usage instructions, threshold guide, project structure

README.md

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+# deduper
+
+Content-based duplicate file detection CLI for Linux.
+
+Finds exact and near-duplicate files using cryptographic hashing and perceptual fingerprinting. Designed for large media libraries where filename or size comparisons aren't enough.
+
+## Phases
+
+| Phase | Status | Method |
+|-------|--------|--------|
+| **Image** | ✅ Complete | SHA-256 + dHash perceptual hashing |
+| **Music** | 🔲 Planned | Audio fingerprinting |
+| **Video** | 🔲 Planned | Video fingerprinting |
+
+## Installation
+
+### From source
+
+```bash
+# Requires Rust 1.85+ (2024 edition)
+git clone https://gitea.ingwaz.work/admin/deduper.git
+cd deduper
+cargo build --release
+```
+
+Binary lands at `target/release/deduper`.
+
+## Usage
+
+```bash
+deduper <folder> [hamming-threshold]
+```
+
+| Argument | Required | Default | Description |
+|----------|----------|---------|-------------|
+| `folder` | yes | — | Root directory to scan (recursive) |
+| `hamming-threshold` | no | `8` | Max Hamming distance for perceptual similarity (0–64) |
+
+### Examples
+
+**Scan a photo library with default threshold:**
+
+```bash
+deduper ~/Pictures
+```
+
+**Strict matching (fewer false positives):**
+
+```bash
+deduper ~/Pictures 4
+```
+
+**Relaxed matching (catches more variants):**
+
+```bash
+deduper ~/Pictures 12
+```
+
+### Output
+
+```
+group 1 [exact]
+  /home/user/Pictures/vacation/beach.jpg
+  /home/user/Pictures/backup/beach.jpg
+
+group 2 [similar]
+  /home/user/Pictures/vacation/sunset.jpg
+  /home/user/Pictures/edited/sunset_cropped.jpg
+  /home/user/Pictures/thumbs/sunset_small.jpg
+```
+
+- **exact** — Byte-identical files (SHA-256 match)
+- **similar** — Perceptually similar images (dHash Hamming distance ≤ threshold)
+
+## How It Works
+
+### Image Phase
+
+1. **Recursive scan** — Walks the directory tree, filtering by image extension (`jpg`, `jpeg`, `png`, `webp`, `bmp`, `gif`, `tif`, `tiff`)
+2. **SHA-256 hashing** — Identifies byte-identical duplicates
+3. **dHash fingerprinting** — Resizes each image to 9×8 grayscale, compares adjacent pixels to produce a 64-bit perceptual hash
+4. **Hamming distance** — Measures bit differences between dHash values. Lower = more similar
+5. **Union-Find grouping** — Clusters similar images into groups, separating exact from perceptual matches
+
+### Threshold Guide
+
+| Threshold | Behavior |
+|-----------|----------|
+| `0` | Exact perceptual match only (identical visual content, different encoding) |
+| `1–4` | Very conservative — catches resizes and minor compression artifacts |
+| `5–8` | **Recommended** — catches resizes, crops, slight edits |
+| `9–12` | Tolerant — may catch heavily edited versions, higher false positive risk |
+| `13+` | Aggressive — will group loosely related images |
+
+## Supported Formats
+
+`jpg` · `jpeg` · `png` · `webp` · `bmp` · `gif` · `tif` · `tiff`
+
+Non-image files are silently skipped.
+
+## Testing
+
+```bash
+cargo test --all
+```
+
+**20 tests** covering:
+
+- **Unit (13):** Hamming distance, image path filtering, dHash determinism, duplicate grouping logic
+- **Integration (7):** Real image fixtures, empty directories, cropped/resized detection, non-image exclusion, subdirectory recursion, single-file edge case, CLI binary output
+
+### Allure Reports
+
+Test results publish to Allure after each run:
+
+```bash
+cargo test --all 2>&1 | tee /tmp/test_output.txt
+python3 /a0/usr/skills/allure-publish/publish.py deduper
+```
+
+## Project Structure
+
+```
+deduper/
+├── Cargo.toml          # Dependencies: image, sha2, walkdir, anyhow
+├── src/
+│   ├── lib.rs          # Core library: scan, hash, group
+│   └── main.rs         # CLI entrypoint
+├── tests/
+│   └── image_phase.rs  # Integration tests
+└── README.md
+```
+
+## Dependencies
+
+| Crate | Purpose |
+|-------|---------|
+| `image` | Image decoding and manipulation |
+| `sha2` | SHA-256 cryptographic hashing |
+| `walkdir` | Recursive directory traversal |
+| `anyhow` | Error handling |
+
+## License
+
+MIT