feat: simplify layout model selection and archive proposals

Changes:
- Replace PP-Structure 7-slider parameter UI with simple 3-option layout model selector
- Add layout model mapping: chinese (PP-DocLayout-S), default (PubLayNet), cdla
- Add LayoutModelSelector component and zh-TW translations
- Fix "default" model behavior with sentinel value for PubLayNet
- Add gap filling service for OCR track coverage improvement
- Add PP-Structure debug utilities
- Archive completed/incomplete proposals:
  - add-ocr-track-gap-filling (complete)
  - fix-ocr-track-table-rendering (incomplete)
  - simplify-ppstructure-model-selection (22/25 tasks)
- Add new layout model tests, archive old PP-Structure param tests
- Update OpenSpec ocr-processing spec with layout model requirements

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

openspec/changes/archive/2025-11-27-simplify-ppstructure-model-selection/tasks.md

@@ -0,0 +1,56 @@
+# Implementation Tasks
+
+## 1. Backend API Changes
+
+- [x] 1.1 Update `app/schemas/task.py` to add `layout_model` enum type
+- [x] 1.2 Update `app/routers/tasks.py` to replace `pp_structure_params` with `layout_model` parameter
+- [x] 1.3 Update `app/services/ocr_service.py` to map `layout_model` to `layout_detection_model_name`
+- [x] 1.4 Remove custom PP-Structure engine creation logic (use model selection instead)
+- [x] 1.5 Add backward compatibility: default to "chinese" if no model specified
+
+## 2. Backend Configuration
+
+- [x] 2.1 Keep `layout_detection_model_name` in `config.py` as fallback default
+- [x] 2.2 Keep fine-tuning parameters in `config.py` (not exposed to API)
+- [x] 2.3 Document available layout models in config comments
+
+## 3. Frontend Changes
+
+- [x] 3.1 Remove `PPStructureParams.tsx` component
+- [x] 3.2 Update `src/types/apiV2.ts`:
+  - Remove `PPStructureV3Params` interface
+  - Add `LayoutModel` type: `"default" | "chinese" | "cdla"`
+  - Update `ProcessingOptions` to use `layout_model` instead of `pp_structure_params`
+- [x] 3.3 Create `LayoutModelSelector.tsx` component with:
+  - Radio buttons or dropdown for model selection
+  - Clear descriptions for each model option
+  - Default selection: "chinese"
+- [x] 3.4 Update task start form to use new `LayoutModelSelector`
+- [x] 3.5 Update API calls to send `layout_model` instead of `pp_structure_params`
+
+## 4. Internationalization
+
+- [x] 4.1 Add i18n strings for layout model options:
+  - `layoutModel.default`: "Standard Model (English documents)"
+  - `layoutModel.chinese`: "Chinese Document Model (Recommended)"
+  - `layoutModel.cdla`: "CDLA Model (Chinese layout analysis)"
+- [x] 4.2 Add i18n strings for model descriptions
+
+## 5. Testing
+
+- [x] 5.1 Create new tests for `layout_model` parameter (`test_layout_model_api.py`, `test_layout_model.py`)
+- [x] 5.2 Archive tests for `pp_structure_params` validation (moved to `tests/archived/`)
+- [x] 5.3 Add tests for layout model selection (19 tests passing)
+- [x] 5.4 Test backward compatibility (no model specified → use chinese default)
+
+## 6. Documentation
+
+- [ ] 6.1 Update API documentation for task start endpoint
+- [ ] 6.2 Remove PP-Structure parameter documentation
+- [ ] 6.3 Add layout model selection documentation
+
+## 7. Cleanup
+
+- [x] 7.1 Remove localStorage keys for PP-Structure params (`pp_structure_params_presets`, `pp_structure_params_last_used`)
+- [x] 7.2 Remove any unused imports/types related to PP-Structure params
+- [x] 7.3 Archive old PP-Structure params test files