feat(react-pdf): support $computed functions and custom directives

[Kavishkagaya]

Summary

@json-render/react-pdf built its resolution context with only stateModel, so every $computed expression resolved to undefined and custom directives were left as unresolved objects. This adds functions and directives support, mirroring @json-render/react.

• VisibilityProvider accepts functions and directives, builds a memoized directive registry with core's createDirectiveRegistry, and puts both in the context
• JSONUIProvider and createRenderer output accept and forward both props
• renderToBuffer / renderToStream / renderToFile accept both options and pass them through renderElement, including repeat children

No changes to core or other renderers.

Tests

• renderer.test.tsx: $computed via JSONUIProvider + Renderer, a custom directive, a directive wrapping $computed, and $computed with $item args per repeat iteration
• render.test.tsx: the same cases through renderToBuffer

The new tests fail without the fix and pass with it. check-types, lint and the react-pdf/core vitest suites pass.

Docs

• directives.mdx: lists React PDF among renderers that accept directives
• api/react-pdf.mdx, package README and skill: document functions / directives
• docs-baseline.json: refreshed hashes for the two edited pages (pnpm --filter web test:routes passes, 122/122)

[vercel]

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