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>

frontend/src/pages/ProcessingPage.tsx

@@ -9,10 +9,10 @@ import { Badge } from '@/components/ui/badge'
 import { useToast } from '@/components/ui/toast'
 import { apiClientV2 } from '@/services/apiV2'
 import { Play, CheckCircle, FileText, AlertCircle, Clock, Activity, Loader2 } from 'lucide-react'
-import PPStructureParams from '@/components/PPStructureParams'
+import LayoutModelSelector from '@/components/LayoutModelSelector'
 import TaskNotFound from '@/components/TaskNotFound'
 import { useTaskValidation } from '@/hooks/useTaskValidation'
-import type { PPStructureV3Params, ProcessingOptions } from '@/types/apiV2'
+import type { LayoutModel, ProcessingOptions } from '@/types/apiV2'
 
 export default function ProcessingPage() {
   const { t } = useTranslation()
@@ -31,8 +31,8 @@ export default function ProcessingPage() {
     },
   })
 
-  // PP-StructureV3 parameters state
-  const [ppStructureParams, setPpStructureParams] = useState<PPStructureV3Params>({})
+  // Layout model state (default to 'chinese' for best Chinese document support)
+  const [layoutModel, setLayoutModel] = useState<LayoutModel>('chinese')
 
   // Start OCR processing
   const processOCRMutation = useMutation({
@@ -40,11 +40,7 @@ export default function ProcessingPage() {
       const options: ProcessingOptions = {
         use_dual_track: true,
         language: 'ch',
-      }
-
-      // Only include pp_structure_params if user has customized them
-      if (Object.keys(ppStructureParams).length > 0) {
-        options.pp_structure_params = ppStructureParams
+        layout_model: layoutModel,
       }
 
       return apiClientV2.startTask(taskId!, options)
@@ -346,11 +342,11 @@ export default function ProcessingPage() {
         </Card>
       )}
 
-      {/* PP-StructureV3 Parameters (only show when task is pending) */}
+      {/* Layout Model Selection (only show when task is pending) */}
       {isPending && (
-        <PPStructureParams
-          value={ppStructureParams}
-          onChange={setPpStructureParams}
+        <LayoutModelSelector
+          value={layoutModel}
+          onChange={setLayoutModel}
           disabled={processOCRMutation.isPending}
         />
       )}