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/2025-11-26-fix-ocr-table-empty-columns/specs/ocr-processing/spec.md

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+## ADDED Requirements
+
+### Requirement: OCR Table Empty Column Cleanup
+
+The OCR Track converter SHALL clean up PP-Structure generated tables by removing columns where all rows have empty or whitespace-only content.
+
+The system SHALL:
+1. Identify columns where every cell's content is empty or contains only whitespace (using `.strip()` to determine emptiness)
+2. Remove identified empty columns from the table structure
+3. Update the `columns`/`cols` value to reflect the new column count
+4. Recalculate each cell's `col` index to maintain continuity
+5. Adjust `col_span` values when spans cross removed columns (shrink span size)
+6. Remove cells entirely when their complete span falls within removed columns
+7. Preserve original bbox and page coordinates (no layout drift)
+8. If `columns` is 0 or missing after cleanup, fill with the calculated column count
+
+The cleanup SHALL NOT:
+- Remove columns where the header is empty but data rows contain values
+- Modify tables in Direct or HYBRID track
+- Alter the original bbox coordinates
+
+#### Scenario: All rows in column are empty
+- **WHEN** a table has a column where all cells contain only empty or whitespace content
+- **THEN** that column is removed
+- **AND** remaining cells have their `col` indices decremented appropriately
+- **AND** `cols` count is reduced by 1
+
+#### Scenario: Column has empty header but data has values
+- **WHEN** a table has a column where the header cell is empty
+- **AND** at least one data row cell in that column contains non-whitespace content
+- **THEN** that column is NOT removed
+
+#### Scenario: Cell span crosses removed column
+- **WHEN** a cell has `col_span > 1`
+- **AND** one or more columns within the span are removed
+- **THEN** the `col_span` is reduced by the number of removed columns within the span
+
+#### Scenario: Cell span entirely within removed columns
+- **WHEN** a cell's entire span falls within columns that are all removed
+- **THEN** that cell is removed from the table
+
+#### Scenario: Missing columns metadata
+- **WHEN** the table dict has `columns` set to 0 or missing
+- **AFTER** cleanup is performed
+- **THEN** `columns` is set to the calculated number of remaining columns
+
+### Requirement: OCR Table Column Alignment by Bbox
+
+(Optional Enhancement) When bbox coordinates are available for table cells, the OCR Track converter SHALL use cell bbox x0 coordinates to improve column alignment accuracy.
+
+The system SHALL:
+1. Sort cells by bbox `x0` coordinate before assigning column indices
+2. Reassign `col` indices based on spatial position rather than HTML order
+
+This requirement is optional and implementation MAY be deferred if bbox data is not reliably available.
+
+#### Scenario: Cells reordered by bbox position
+- **WHEN** bbox coordinates are available for table cells
+- **AND** the original HTML order does not match spatial order
+- **THEN** cells are reordered by `x0` coordinate
+- **AND** `col` indices are reassigned to reflect spatial positioning