bench: vision OCR accuracy baseline (#416)

[Supermonters]

Closes #416

Mục tiêu

Đo CER/WER vision OCR trên corpus scan tiếng Việt; so với baseline Tesseract historical (bench/REPORT.md §4). Chỉ đo & report — không đổi prompt/model.

Thay đổi

• Thêm bench/REPORT_OCR_VISION.md — kết quả fileconv accuracy (5 mẫu).

Terminal evidence

Xem comment đầy đủ trên #416 (format inter-v2-07). Tóm tắt:

File	Label	CER	Accuracy	ms
vn_printed.png	image-print-OCR	0.013	98.7%	5454
vn_lowres.png	image-lowres-OCR	0.053	94.7%	5300
vn_hand_caveat.png	handwrite-OCR	0.150	85.0%	6371
vn_hand_dancing.png	handwrite-OCR	0.005	99.5%	5133
gold-004.pdf	pdf-scan-OCR	0.111	88.9%	4579

TB 5 mẫu: CER ~0.066 (~93.4% accuracy); ms/file TB ~5367.

Contrast Tesseract (historical) vs vision

Nguồn: bench/REPORT.md §4 (2026-06-30). Tesseract removed 2026-08-10.

Kịch bản	Tesseract (historical)	Vision (2026-08-21)
image-print-OCR	99.2%	98.7%
image-lowres-OCR	99.0%	94.7%
handwrite-OCR (TB)	~47.9%	92.2%
pdf-scan	~1200 ms/trang	88.9% / ~4.6 s

Lệnh recreate

bash bench/make_vn_images.sh          # macOS: DejaVu ~/Library/Fonts/, dùng magick
bash bench/download_pdfium.sh
export FILECONV_OCR_API_KEY=...

cat > bench/vn_corpus/manifest_ocr.tsv <<'TSV'
# file	ground_truth	label
vn_printed.png	ref.txt	image-print-OCR
vn_lowres.png	ref.txt	image-lowres-OCR
vn_hand_caveat.png	ref.txt	handwrite-OCR
vn_hand_dancing.png	ref.txt	handwrite-OCR
../markhand_web/golden/documents/gold-004.pdf	../markhand_web/golden/markdown/gold-004.md	pdf-scan-OCR
TSV

cargo build --release -p fileconv-cli
./target/release/fileconv accuracy bench/vn_corpus/manifest_ocr.tsv bench/REPORT_OCR_VISION.md

macOS note: make_vn_images.sh upstream hardcode Linux font path; local workaround: DEJAVU=$HOME/Library/Fonts/DejaVuSans.ttf + magick (không commit trong PR này).

Acceptance checklist

• Benchmark ≥5 samples
• Report CER, ms/file
• Bảng vision vs Tesseract (historical)
• Observations tiếng Việt (comment [inter-v2-09] Benchmark OCR vision vs baseline Tesseract #416)
• Không commit corpus/PNG/key

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