feat(web): render web_search/web_fetch output as a web card

Consume the card:'web' result view (structured sources + answer for search, the
URL and HTTP status for fetch) the web backend PR added. WebBlock
(ui-primitives) draws both kinds via the kind discriminant: a citation list of
safe external links (the MarkdownText protocol allowlist, title-or-hostname
label), a truncation indicator, a height cap matching TerminalBlock; a fetch
summary for the other kind. web-card-model is the single resultView derivation;
a keyed WebRow registers under web_search and web_fetch with the card resident
under its summary. The generic fallback and the details panel are web-aware.
Fixture gains web_search and web_fetch turns for the built-boot snapshot.

packages/client/ui-primitives/src/index.ts

@@ -22,6 +22,8 @@ export { JsonTree } from './JsonTree.tsx'
 export type { JsonTreeProps } from './JsonTree.tsx'
 export { TerminalBlock, DEFAULT_TERMINAL_MAX_LINES } from './TerminalBlock.tsx'
 export type { TerminalBlockProps } from './TerminalBlock.tsx'
+export { WebBlock, DEFAULT_WEB_MAX_SOURCES } from './WebBlock.tsx'
+export type { WebBlockProps, WebSearchBlockProps, WebFetchBlockProps, WebSourceView } from './WebBlock.tsx'
 export { CodeBlock } from './markdown/CodeBlock.tsx'
 export { JsonBlock } from './markdown/JsonBlock.tsx'
 export { MarkdownText } from './markdown/MarkdownText.tsx'