add --web-snapshot-viewer mode

[maagenta]

Adds a new exclusive proxy mode that renders modern web pages server-side using Playwright and serves them to old browsers as interactive strip images.

When started with --web-snapshot-viewer, all HTTP requests are routed through WebSnapshotViewer instead of the normal proxy flow. The flow is:

1. Browser requests any URL → proxy returns a lightweight probe page (JS)
2. JS reads viewport dimensions and redirects to snapshot.webone.internal/snap
3. Playwright takes a full-page PNG screenshot at the requested viewport size
4. Screenshot is split into horizontal strips (default 100px height each)
5. Shell page is returned with tags — only the first visible strips load
6. As the user scrolls, strips load lazily via JS
7. Clicks are forwarded to Playwright, changed strips are updated in place

• WebSnapshotViewer.cs: HTTP router for all snapshot endpoints

• DimensionProbe.cs: generates the probe page for viewport detection

• ScreenshotEngine.cs: Playwright wrapper (screenshot, scroll, click)

• SnapshotPage.cs: generates the HTML shell page with strip images

• StripManager.cs: splits PNG into strips, hashes each for change detection

• BlankStrip.cs: generates per-session checkerboard GIF placeholder

• ScrollHandler.cs: inline scroll sync JS + /scroll-pos endpoint

• ClickHandler.cs: click overlay JS + /click endpoint

• ClientStripManager.cs: shared JS helpers (sendCmd, updateStrip, reloadPage)

• ARCHITECTURE.md: full system documentation

• --web-snapshot-viewer: enable the mode

• --snapshot-headed: run Playwright browser in visible (headed) mode

• --quality N: JPEG quality 1-100 (default 85)

• --strip-size N: strip height in pixels (default 100)

• --set-min-threads N: thread pool minimum (default 1000)

• JPEG encoded as RGB colorspace to avoid YCbCr->RGB AltiVec crash in Safari 1 on Mac OS X 10.3 PPC (vec_ycc_rgb_convert bug)

• Lazy strip loading: only first viewport/stripHeight+2 strips load initially

• Blank strip placeholder: per-session checkerboard GIF instead of black/white

• Scroll sync via new Image() fire-and-forget (no XHR, Safari 1 compatible)

• Click transport via hidden cmd iframe (Safari 1 compatible AJAX equivalent)

• Thread pool raised to 1000 to avoid starvation with many concurrent connections

• Browser pre-warmed at startup so first request does not wait for Playwright

• Microsoft.Playwright 1.*

• SixLabors.ImageSharp 3.*