feat(playground): reuse the landing PSP model

[doodlewind]

Summary

• Reuse the landing page Pocket Stage adapter, authored PSP profile, and GLB around the Playground live framebuffer.
• Keep the Playground PocketHost as the single runtime owner while the shared Stage supplies responsive 3D raycast controls, keyboard focus, attribution, and a 2D fallback.
• Integrate main at ad6b638 so its Cursor, Launcher, Octane, runtime export, and full-demo validation fixes remain active alongside the PSP shell.
• Preserve deterministic 28-variant verification by driving the shared PocketHost directly while the Stage verifier separately covers real PSP raycasts and WebGL uploads.
• Release pending model input before live recompilation resets the host, including guest-error cleanup.
• Bind both split Vue Vapor browser bundles to the PocketJS document facade so text nodes mount into the native tree.
• Make the spinner SVGs valid standalone browser images and normalize future namespace-less SVG assets in the Playground baker.

Validation

• bun run site:verify-playground — 28/28 demo/framework variants passed deterministic real-browser input comparisons
• bun run test — 11/11 stages green
• bun test tests/site-stage.test.ts — 10 pass, 0 fail, 111 assertions
• bunx tsc --noEmit
• git diff --check
• Solid Stage verifier — complete shared GLB, one WASM and zero launcher requests, reset-safe raycast input, 8 distinct framebuffer animation hashes, 8 distinct WebGL upload hashes, and no browser errors
• Vue Vapor verifier — complete shared GLB, 17 inserted native text nodes, zero invalid text writes or inserts, one WASM and zero launcher requests, and no browser errors
• Visual Chrome checks for the corrected Solid and Vue Vapor PSP screens