[codex] Add spread v2 image watermark payload mode

[CEQ151]

Summary

• add frequency-spread-v2 image watermarking with keyed fingerprint64 payloads by default
• keep text16 compatibility mode and route low-visibility profiles through the new DWT/DCT spread codec
• surface payload mode, fingerprint, confidence, BER, and codec diagnostics through helper, IPC, and UI
• add spread-v2 unit tests and a local benchmark runner for clean/JPEG/resize/blur/crop/FPR checks

Validation

• ..venv-ml\Scripts\python -m unittest discover -s blind_watermark\tests
• npm run build
• npx eslint --quiet src/main/index.ts src/preload/index.ts src/preload/index.d.ts src/renderer/src/components/ImageWatermarkPanel.tsx src/renderer/src/i18n.ts

Notes

• Draft PR because resources/bin/bwm_helper.exe has not been rebuilt yet; packaged production builds still need a helper rebuild before release.
• Local smoke on 5 train images: clean/JPEG85 20/20, original FPR 0/20, PSNR >= 51.331 dB, SSIM >= 0.997982.