Bundle MRD Viz release handoff with D3 metadata groups

[ShiyunXu]

Summary

• Fold D3 into the researcher rollout PR instead of keeping it separate
• Add the requested before/after rollout table covering D1, Release, D3, D2, Marketplace, and F10
• Add a stable install entry point in mrd-viz/README.md: https://github.com/ismrmrd/mrd/releases/latest
• Add the D3 metadata grouping scaffold: a typed file/header/image/acquisition/stream group model and a new default Metadata → Groups tab
• Document the setup commands in the extension README so the installed-release path is easier to follow

UX flow captured by this PR

1. Researchers install the platform VSIX from the latest GitHub Release page.
2. Opening .mrd uses the bundled backend when available, or guides users to automatic/backendPath setup when needed.
3. Once a file opens, metadata starts in the grouped D3 view instead of forcing users to scan raw stream/JSON first.

Validation

• npm run compile
• npm run lint
• xvfb-run -a npm test

[ShiyunXu]

Closing: the researcher install story is landing as a single approval in #91, which already contains D1, D3 (PyInstaller standalone backend + platform VSIX matrix), and the release workflow together. Keeping this open would re-separate D3 from the install path, which is the split we decided against.

One piece of real work here is not carried over: the metadata grouping in media/viewer/metadataGroups.ts (214 lines + tests). That is a genuine feature, but it is not D3 — D3 is the zero-prerequisite bundled-backend install. Grouped metadata is Stage 2 "richer metadata display" per TECHNICAL_DESIGN.md.

It should be re-filed as its own feature PR against carter-mrd-viz after the release lands, so it is reviewed as a viewer feature and does not gate getting a build into researchers hands.