feat(design-system): add portable open-source components

[mahanti]

Reusable open-source components

Adds @buzz/ui: Buzz-owned visual treatments over public Base UI behavior, with public calendar and resize primitives. Inter and Lucide replace private brand assets. Source and compiled CSS entry points allow the library to run outside the desktop shell or an internal documentation host.

Includes actions, form controls, selection, menus, overlays, navigation, feedback, layout, a categorical bar chart, and a manual carousel. Named variants, state-aware classes, accessible labels, focus, reduced motion, and upstream license notices are part of the package contract. The following stack layer adds the complete interactive catalog and browser validation.

Stack 2 of 3: #7522 → this PR → #7525. Base: #7522. All three PRs remain draft.

Validation: shared library build/type declarations, Buzz consumer production build, token regeneration, typography audit, contrast audit, and token compiler tests pass. just ci was run and hits the existing timing-sensitive buzz-acp::acp::tests::keepalive_resets_idle_past_deadline failure; that test passes in isolation. The same run also hit idle_resets_on_stdout_activity. Final full-stack validation, including the full passing ACP suite with serialized tests, is recorded in #7525. No ACP code is changed.