Build the shared Base UI control family

[mahanti]

Shared controls now follow the approved Block UI direction while retaining Base UI behavior: 32/40/52px pill buttons, named emphasis and destructive styles, loading protection, and shared Field/Input/Textarea/RadioGroup/Checkbox components. SearchField forwards input refs and keyboard handlers and restores focus after clearing. Existing Button names remain compatible while callers migrate.

Stack 2/4: #73 → #85 → this PR → #87 → #88. Controls use semantic colors in both modes. Inter and JetBrains Mono remain; no private package, font, artwork or business examples are added. Avatars retain their independent 24/32/40px sizes.

Validation: TypeScript, viewer build, design guards and 47 design tests passed. The completed stack also passed all 26 viewer browser checks. Mounted tests cover loading activation/submission, field associations, radio keyboard and form behavior, disabled choices, and search focus. Radio options use Base UI Field.Item to preserve their individual labels and IDs. No browser cases are added or removed.

A separate test-only follow-up prepares the existing 1,025/1,024 signed capacity fixtures in scoped setup. Both capacity checks retain real signatures, every record, all assertions and the unchanged timeout settings. The earlier one-worker full run passed 1,561/1,563 tests, with only these signing-heavy cases timing out at 5.53/5.80 seconds. The focused file now passes all 19 tests (7.57 seconds total, including preparation; behavior assertions 14/15ms). These are local macOS measurements under different machine load, not a speedup claim. No relay production code changes.

Hosted CI is required on the final branch head. Full local just scan, native visual testing and human review remain deferred; this is a draft, not integration-ready.