Add element-box fusion and reading-order for set-of-marks

[JE-Chen]

set_of_marks.mark_elements numbers a single clean element list — but nothing produced it. A real screen parse yields three overlapping sources (OCR / icon / a11y boxes) with duplicates and no order. This is the missing connective tissue.

• iou / merge_boxes (AC_… via fuse): IoU overlap of two boxes; drop near-duplicates above a threshold (largest kept).
• fuse_elements (AC_fuse_elements): union OCR + icon + a11y boxes, tagging each with its source and dropping cross-source overlaps by source_priority (default a11y > ocr > icon, then larger area).
• reading_order (AC_reading_order): band rows within row_tol px, order by x within each row, return new dicts with a stable index — ready to feed set_of_marks.
• Plain dict boxes (x,y,width,height + extras) → pure-stdlib, fully headless-testable. Wired through all 5 layers + headless test + EN/Zh docs + WHATS_NEW. Qt-free.

[codacy-production]

Not up to standards ⛔

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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