docs: add usage and verification notes

README.md

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+# Star Chart Search Enhancer
+
+Chrome MV3 extension for the Xingtu creator market page.
+
+## Development
+
+```bash
+npm install
+npm test
+npm run build
+```
+
+## Load The Extension
+
+1. Run `npm run build`
+2. Open `chrome://extensions`
+3. Enable developer mode
+4. Choose `Load unpacked`
+5. Select the `dist/` directory
+
+## Current Scope
+
+- Adds two after-search-rate columns to the Xingtu market list
+- Hydrates the current page immediately
+- Provides plugin-owned filter, sort, and CSV export controls
+- Triggers full-scan flow only when filter, sort, or export is used
+
+## Manual Verification
+
+1. Load the unpacked extension from `dist/`
+2. Open `https://xingtu.cn/ad/creator/market`
+3. Confirm the two new columns appear
+4. Confirm current-page rows move through loading and then render values or failure states
+5. Apply a threshold filter and confirm the list hides unmatched rows
+6. Apply a sort and confirm row order changes
+7. Export CSV and confirm the file includes plugin status and after-search-rate fields