feat(shared): establish canonical WebP screenshot pipeline

[quanruzhuoxiu]

Summary

• add a shared PNG/JPEG/WebP format contract and canonicalize static screenshots to WebP
• use Sharp in Node.js and Canvas/OffscreenCanvas in browsers while preserving native WebP and explicitly preserved JPEG streams
• propagate MIME type and file extension through screenshot identity, storage, HTML reports, split reports, Markdown export, Report App, and Visualizer
• retain the exact post-processing image bytes sent to the model and associate them with report evidence by SHA-256
• document WebP input support in both English and Chinese API references

PR stack

This is PR 1 of 3 and targets main.

The browser-producer and device-producer PRs will target this branch. The original PR #2826 and feat/chrome-extension-default-webp branch remain unchanged as a rollback option.

Validation

• pnpm --filter @midscene/shared test (33 files, 433 tests)
• pnpm --filter @midscene/core test (125 files, 1357 passed, 8 skipped)
• pnpm --filter @midscene/report test (16 files, 87 tests)
• pnpm --filter @midscene/visualizer test (21 files, 104 tests)
• pnpm exec nx run-many -t build -p @midscene/shared @midscene/core @midscene/report @midscene/visualizer
• pnpm exec nx run-many -t build -p @midscene/android-playground @midscene/computer-playground @midscene/ios @midscene/harmony
• pnpm run type-check:tests
• pnpm run lint

[cloudflare-workers-and-pages]

Deploying midscene with    Cloudflare Pages

Latest commit:	46daa47
Status:	✅  Deploy successful!
Preview URL:	https://d757d766.midscene.pages.dev
Branch Preview URL:	https://feat-webp-screenshot-foundat.midscene.pages.dev

View logs

[quanruzhuoxiu]

Closing this superseded WebP stack PR. The replacement stack is #3012 → #3013 → #3014 and incorporates the latest main branch plus the follow-up architecture, format-validation, and CI fixes. PR #2856 remains open because it is an independent model-input evidence change.