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/fix-ocr-track-table-rendering/specs/pdf-generation/spec.md

@@ -1,52 +0,0 @@
-# PDF Generation - OCR Track Table Rendering Fix
-
-## MODIFIED Requirements
-
-### Requirement: OCR Track Table Content Conversion
-
-The PDF generator MUST properly convert table content from JSON dict format to renderable structure when processing OCR track results.
-
-#### Scenario: Table dict with cells array converts to proper HTML
-
-Given an OCR track JSON with table element containing rows, cols, and cells array
-When the PDF generator processes this element
-Then the table content MUST be converted to a TableData object
-And TableData.to_html() MUST produce valid HTML with proper tr/td structure
-And the generated PDF table MUST have cells positioned in correct grid locations
-
-#### Scenario: Table with rowspan/colspan renders correctly
-
-Given a table element with cells having rowspan > 1 or colspan > 1
-When the PDF generator renders the table
-Then merged cells MUST span the correct number of rows/columns
-And content MUST appear in the merged cell position
-
-### Requirement: Table Visual Fidelity
-
-The PDF generator MUST render OCR track tables with visual structure matching the original document.
-
-#### Scenario: Table renders with grid lines
-
-Given an OCR track table element
-When rendered to PDF
-Then the table MUST have visible grid lines/borders
-And cell boundaries MUST be clearly defined
-
-#### Scenario: Table text alignment preserved
-
-Given an OCR track table with cell content
-When rendered to PDF
-Then text MUST be positioned within the correct cell boundaries
-And text MUST NOT overflow into adjacent cells
-
-### Requirement: Backward Compatibility with Hybrid Mode
-
-The table rendering fix MUST NOT break hybrid mode processing.
-
-#### Scenario: Hybrid mode tables render correctly
-
-Given a document processed with hybrid mode combining Direct and OCR tracks
-When PDF is generated
-Then Direct track tables MUST render with existing quality
-And OCR track tables MUST render with improved quality
-And no regression in table positioning or content