Tighten Pro page, fix demo inconsistencies, enable Mermaid diagrams

Rework the Pro marketing page for consistency and concision, and turn on
Mermaid rendering across the docs.

Pro page (pro.tsx / pro.module.css):
- Replace the three-tile proof section with two real demos, each linking to
  both a live app and its source: LocalHub (RSC performance) and the new
  TanStack starter. The old layout split LocalHub's live demo and its repo
  into two separate tiles and showcased the spec/dummy CI fixture as an
  example app.
- Drop the spec/dummy CI test app from the marketing page.
- Cut repeated licensing / build-free copy that appeared up to five times.
  The hero highlight now covers the free side and the license card the paid
  side, with no overlap.
- Merge Immediate hydration into Streaming SSR so the feature grid is a
  balanced 3x2 (6 cards) instead of 7 with an orphan.
- Reduce three competing hero CTAs to two; the live demo now lives in the
  demo section.

examples.tsx:
- Add the TanStack starter (live demo + source repo), the strongest current
  example, which was missing from the page.

Enable Mermaid (docusaurus.config.ts / package.json / lockfile):
- Add @docusaurus/theme-mermaid and markdown.mermaid. This renders the
  existing RSC rendering-flow sequence diagram, which previously shipped as a
  raw code block, and unblocks diagrams synced from the upstream docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: justin808

prototypes/docusaurus/docusaurus.config.ts

@@ -58,6 +58,7 @@ const config: Config = {
 
   onBrokenLinks: 'warn',
   markdown: {
+    mermaid: true,
     hooks: {
       onBrokenMarkdownLinks: 'warn',
     },
@@ -68,7 +69,7 @@ const config: Config = {
     locales: ['en'],
   },
 
-  themes: useAlgolia ? [] : [localSearchTheme],
+  themes: [...(useAlgolia ? [] : [localSearchTheme]), '@docusaurus/theme-mermaid'],
 
   presets: [
     [