Add live node kind previews - #38

Background

Users need to see what each Stack node kind actually looks like, including the approved rounded cache fallback, without relying on stale pre-generated SVGs. The Web renderer also needs to consume the released Engine 0.9.0 layout improvements and the final canonical Docs bundle.

Implementation Approach

• Add an accessible live Node Kind catalog for all ten semantic kinds with default, light, and dark theme controls.
• Render every preview locally through @stack-sh/engine 0.9.0 WASM, expose the exact Engine/Theme metadata, and copy kind <name> syntax from keyboard-accessible cards.
• Revoke Blob URLs and guard delayed Engine/clipboard work during unmount.
• Pin the merged stack-sh/docs commit 16b8ac60219f5051bdb0319048e9b6200d3a614a and its exact manifest SHA-256.

Testing Approach

• 39 Vitest tests and 28 Docs contract tests.
• 60 localized documentation pages, 11 complete Docs examples, and nine canonical gallery examples rendered with Engine 0.9.0.
• Playground and VitePress production builds, output validation, Cloudflare dry run, and 21 localized examples exercised against Stack CLI 0.5.3.
• Prior Google Chrome QA covered all ten live cards, theme switching, keyboard copy, console cleanliness, and 320/768/1024/1440 px layouts; production is rechecked after deployment.

Review Perspectives / Points of Concern

• No preview SVG is checked in. Every card is rendered on-device from canonical Stack source so later Engine pins update the visuals.
• kind remains semantic; shape and palette remain theme-owned, and custom cylinder themes are still supported.
• The catalog loads Engine lazily and cleans up all generated object URLs. It does not add network rendering or remote source upload.