Add revealRate reveal pacing to <Renderer> (#751)

[oakleaf]

Implements the reveal-pacing option proposed in #751.

Problem

<Renderer> renders progressively as the response arrives, so arrival rate is the reveal pace. When the DSL arrives bursty or whole — a fast tool-call generation, or a transport that coalesces writes (serverless response streaming buffers small writes, CDNs re-clump) — the components pop in all at once instead of assembling.

Approach

A new revealRate prop. While isStreaming, the Renderer feeds the streaming parser a growing prefix of the response on a clock, so completed statements stay stable (the parser already caches them) while the pending one fills in — the UI assembles the way it would over a smooth stream. It isn't fighting the parser; it's set() used as designed, paced on a timer instead of on network arrival.

<Renderer response={dsl} isStreaming revealRate />
<Renderer response={dsl} isStreaming revealRate={{ targetMs: 1100 }} />

• Default off — revealRate omitted/false keeps the current behavior. Fully backward compatible.
• Count-based and never resets mid-stream, so a non-append-only DSL change (a new element growing the root's child-ref list) keeps typing forward instead of blanking; the parser reconciles the shifted prefix by statementId.
• Settled content and prefers-reduced-motion render immediately.

Structure

• reveal.ts — pure pacing math (framework-free, unit-tested).
• hooks/useRevealedResponse.ts — thin requestAnimationFrame + reduced-motion glue.
• Renderer.tsx — new revealRate prop, paces response before useOpenUIState.
• Docs: api-reference/react-lang.mdx gains the prop + a "Reveal pacing" section.

Tests

• reveal.test.ts — the pacing math.
• reveal-integration.test.ts — drives the real createStreamingParser with clock-paced prefixes and asserts the tree genuinely assembles (the root renders mid-stream, before every statement has completed) and never blanks a completed statement, landing exactly where a full parse does.
• All of react-lang's checks pass (test, typecheck, lint:check, format:check, build). No new dependencies.

Happy to adjust the API shape (reveal="smooth", different defaults, statement- vs char-granularity) to match your preference.

Closes #751