[TheGreatAxios]

Summary

Stacked on #69. Merge after it.

• 45 ui/ and blocks/ modules start with "use client": the ones that call a hook, create a context, define an inline event handler, or import an optional peer. SWC keeps the directive in dist/, so a React Server Components app imports them without its own client wrapper. Outside RSC the directive is a no-op. Vite logs the same "module level directives" warning it logs for Radix.
• dep-guard rule 3 fails when a component's directive does not match what it does. It builds the peer pattern from OPTIONAL_PEERS, ignores comments, and catches generic hook calls (useState<T>(). After a build it also fails when a marked module's dist/ file lost the directive, and bun run build now ends with dep-guard.
• The README, ARCHITECTURE and CONTRIBUTING describe the rule.
• New stateful components in feat(ui): primitives for the stage-workspace port #58/feat(chat): optional attachIcon and sendIcon on ChatInput #59 need the directive once this lands. dep-guard flags them.

Verification

typecheck, lint, dep-guard, bun test and bun run build pass. Deleting the directive from a built dist/ui/switch.js makes dep-guard fail. A Vite production build and the Playwright suite pass against the packed tarball of the merged stack.

Closes CL-9103