# Change: Simplify PP-StructureV3 Configuration with Layout Model Selection

## Why

Current PP-StructureV3 parameter adjustment UI exposes 7 technical ML parameters (thresholds, ratios, merge modes) that are difficult for end users to understand. Meanwhile, switching to a different layout detection model (e.g., CDLA-trained models for Chinese documents) would have a much greater impact on OCR quality than fine-tuning these parameters.

**Problems with current approach:**
- Users don't understand what `layout_detection_threshold` or `text_det_unclip_ratio` mean
- Wrong parameter values can make OCR results worse
- The default model (PubLayNet-based) is optimized for English academic papers, not Chinese business documents
- Model selection is far more impactful than parameter tuning

## What Changes

### Backend Changes
- **REMOVED**: API parameter `pp_structure_params` from task start endpoint
- **ADDED**: New API parameter `layout_model` with predefined options:
  - `"default"` - Standard model (PubLayNet-based, for English documents)
  - `"chinese"` - PP-DocLayout-S model (for Chinese documents, forms, contracts)
  - `"cdla"` - CDLA model (alternative Chinese document layout model)
- **MODIFIED**: PP-StructureV3 initialization uses `layout_detection_model_name` based on selection
- Keep fine-tuning parameters in backend `config.py` with optimized defaults

### Frontend Changes
- **REMOVED**: `PPStructureParams.tsx` component (slider/dropdown UI for 7 parameters)
- **ADDED**: Simple radio button/dropdown for layout model selection with clear descriptions
- **MODIFIED**: Task start request body to send `layout_model` instead of `pp_structure_params`

### API Changes
- **BREAKING**: Remove `pp_structure_params` from `POST /api/v2/tasks/{task_id}/start`
- **ADDED**: New optional parameter `layout_model: "default" | "chinese" | "cdla"`

## Impact

- Affected specs: `ocr-processing`
- Affected code:
  - Backend: `app/routers/tasks.py`, `app/services/ocr_service.py`, `app/core/config.py`
  - Frontend: `src/components/PPStructureParams.tsx` (remove), `src/types/apiV2.ts`, task start form
- Breaking change: Clients using `pp_structure_params` will need to migrate to `layout_model`
- User impact: Simpler UI, better default OCR quality for Chinese documents
