chore(playground): add browser-only IDE for writing and exporting a Forge game

[stormmuller]

Summary

Adds /playground, a bare-bones, browser-only IDE for writing a small Forge game and downloading it as a self-contained index.html + game.js zip - no backend involved, matching the requested stack (Vite + Monaco + @typescript/vfs + esbuild-wasm, no React).

• Monaco is configured with Forge's real type declarations registered as extra libs (src/editor/forge-types.ts), so autocomplete, hover, and inline diagnostics work against Forge's actual public API (@forge-game-engine/forge/<subpath> imports).
• @typescript/vfs powers an independent, in-browser TypeScript language service (src/editor/type-check.ts) that type-checks the game source before every build - esbuild only strips types, it never checks them.
• esbuild-wasm bundles the game through a custom resolver plugin (src/build/forge-esbuild-plugin.ts) that resolves @forge-game-engine/forge/<subpath> imports - plus Forge's own internal relative imports and its one runtime dependency (seedrandom) - against Forge's compiled dist output, embedded into the app bundle at build time via import.meta.glob. Forge is inlined into the downloaded game rather than referenced externally.
• fflate zips the generated index.html and bundled game.js and triggers a browser download.

/playground is a separate npm project (its own package.json/node_modules), the same pattern as /documentation-site, depending on @forge-game-engine/forge via a file:.. link resolved through dist. See playground/README.md and the new "Browser Playground" section in AGENTS.md for the full architecture and gotchas.

Titled chore (excluded from the changelog requirement) since this adds a repo-internal dev tool with no effect on the published @forge-game-engine/forge package's API or behavior.

Related issue(s)

N/A

Verification checklist

• npm run check-types passes with 0 errors
• npm test passes
• npm run lint passes with 0 errors
• npm run cspell passes with 0 errors
• npm run check-exports passes
• No public API added/changed under /src; not applicable
• No documented engine behavior changed; not applicable
• Not applicable - no /src module with an existing demo was touched
• playground/ verified independently: npm run typecheck (from playground/) passes with 0 errors, and the full flow (type-check → bundle → zip → download) was exercised end-to-end via Playwright - typed the default game in the editor, clicked Build, extracted the downloaded zip, and confirmed it renders and animates correctly in a real browser.

Changelog

• Not applicable - PR titled with an excluded type (chore) since this change has no effect on the published package.

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