refactor(forge): migrate experiments to concept taxonomy

[fronzec]

Summary

• Move the Qt Quick and Qt Widgets experiments into desktop-ui-architecture.
• Move the TMDB client and cached weather experiments into external-service-integration.
• Update the root experiment catalog, concept links, and migration status.

Commits

• fc58989 refactor(forge): migrate desktop UI experiments
• 7274ef9 refactor(forge): migrate external service experiments

Verification

• Python sources in the desktop slice compiled successfully.
• TMDB: go test ./... and go build ./... passed.
• Cached weather: go test ./... and go build ./... passed.
• All moved files were detected as R100 renames; no legacy path references remain.
• No live API, Redis, or GUI runtime was required.

Scope

This PR changes organization and documentation only. It does not touch the separate terminal-ui worktree or its ongoing work.