feat(website): run the playground on the checkout's own engine

[pkurcx]

Summary

• The website no longer pins the published @avsystem/openapi-ng and @avsystem/openapi-ng-wasm32-wasi packages. Vite aliases @avsystem/openapi-ng/browser to the root lib/browser.js, and bundle-engine.mjs copies the wasm loader, worker and binary from the repo root.
• A release is therefore just the vX.Y.Z commit: no follow-up commit bumping the website pins, and PRs that add generator options no longer fail the Docs e2e job.
• The Docs workflow builds the wasm32-wasip1-threads target on every push and PR (cargo cache shared with the CI matrix entry).
• Deploy gate: production only for a release commit (same HEAD check as the publish job) or a docs-only push (website/, docs.yml, bun.lock, bunfig.toml, stackblitz/); manual runs always deploy. Other pushes to main upload a Cloudflare preview tagged main-<sha>, PRs keep their pr-<n> previews.
• Preview builds label the playground engine vX.Y.Z+<sha> so they are distinguishable from production.
• README: the bun link dance is replaced by the two commands needed before running the site.

Trade-offs

• Docs job takes a few minutes longer for the Rust toolchain and wasm build.
• On a release push, Docs and CI publish run in parallel, so the site can go live shortly before the npm package.
• A docs-only hotfix after a release builds the engine from main, so it may carry unreleased generator changes.

Test plan

• bun run build, bun run check, bun run test in website/ pass locally against a freshly built root wasm (16 e2e tests).
• astro dev generates through the alias; cold-cache "Outdated Optimize Dep" 504s are pre-existing (identical count with the old config).
• Gate logic dry-run against real commit ranges.
• This PR's Docs run built the wasm (cargo cache shared with CI restored, 8s compile) and uploaded the pr-9 preview with engine v0.6.0+15303df: https://6ab2fb2e-openapi-ng-docs.pkurc.workers.dev/playground/

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