# Change: Fix Table Column Alignment with Header-Anchor Correction

## Why

PP-Structure's table structure recognition frequently outputs cells with incorrect column indices, causing "column shift" where content appears in the wrong column. This happens because:

1. **Semantic over Geometric**: The model infers row/col from semantic patterns rather than physical coordinates
2. **Vertical text fragmentation**: Chinese vertical text (e.g., "报价内容") gets split into fragments
3. **Missing left boundary**: When table's left border is unclear, cells shift left incorrectly

The result: A cell with X-coordinate 213 gets assigned to column 0 (range 96-162) instead of column 1 (range 204-313).

## What Changes

- **Add Header-Anchor Alignment**: Use the first row (header) X-coordinates as column reference points
- **Add Coordinate-Based Column Correction**: Validate and correct cell column assignments based on X-coordinate overlap with header columns
- **Add Vertical Fragment Merging**: Detect and merge vertically stacked narrow text blocks that represent vertical text
- **Add Configuration Options**: Enable/disable correction features independently

## Impact

- Affected specs: `document-processing`
- Affected code:
  - `backend/app/services/table_column_corrector.py` (new)
  - `backend/app/services/pdf_generator_service.py`
  - `backend/app/core/config.py`

## Problem Analysis

### Example: scan.pdf Table 7

**Raw PP-Structure Output:**
```
Row 5: "3、適應產品..." at X=213
       Model says: col=0

Header Row 0:
  - Column 0 (序號): X range [96, 162]
  - Column 1 (產品名稱): X range [204, 313]
```

**Problem:** X=213 is far outside column 0's range (max 162), but perfectly within column 1's range (starts at 204).

**Solution:** Force-correct col=0 → col=1 based on X-coordinate alignment with header.

### Vertical Text Issue

**Raw OCR:**
```
Block A: "报价内" at X≈100, Y=[100, 200]
Block B: "容--"   at X≈102, Y=[200, 300]
```

**Problem:** These should be one cell spanning multiple rows, but appear as separate fragments.

**Solution:** Merge vertically aligned narrow blocks before structure recognition.
