docs: recommend Pyodide playground path

[lbliii]

Summary

• evaluate the current Pyodide/WASM and Chirp/Railway playground paths
• recommend a Pyodide-first static playground now that Pyodide 314 ships CPython 3.14
• define a half-day compatibility gate, a 2–3 day browser MVP, safety boundaries, proof requirements, effort estimates, and the server-side fallback

Closes #170.

Key finding

The issue's original browser-runtime risk has changed: stable Pyodide 314.x now uses CPython 3.14, while Kida 0.10.0 publishes a dependency-free py3-none-any wheel. A module Web Worker can therefore remove the Python-install funnel without creating an untrusted public render service.

The plan does not claim proven compatibility yet. Outbound npm/CDN requests timed out in this audit environment, so installing and rendering with the published wheel remains the first explicit implementation gate.

Verification

• make lint
• make ty
• make docs (build completes; reports the existing Bengal /llms.txt false-positive class)
• git diff --cached --check
• evidence checked against current Pyodide, PyPI, Railway/Railpack, and chirp-ui sources linked from the recommendation

Steward Notes

• Consulted: planning steward and Railway deployment workflow
• Accepted: browser-first static architecture, JSON-only context, module worker, worker replacement for timeouts, and Chirp/Railway as a follow-up fallback
• Deferred: public deployment, new APIs/config, collaboration, persistence, arbitrary Python execution, and free-threading claims
• Safety: SandboxedEnvironment is explicitly treated as defense-in-depth, not an isolation boundary
• Required proof: cross-browser compatibility gate, cold/warm measurements, diagnostics/render coverage, timeout recovery, CSP/preview review, and browser smoke tests
• No runtime collateral: this PR adds a research artifact only and changes no Kida behavior, public API, dependencies, CLI, schemas, examples, or deployment configuration

[Copilot]

Pull request overview

Adds a one-page research recommendation (closing #170) for how to build a “try Kida” playground without requiring local Python installation, favoring a Pyodide-first static approach with a documented Chirp/Railway fallback.

Changes:

• Introduces a feasibility/recommendation document outlining architecture, safety boundaries, phased effort estimates, and proof gates for a Pyodide module-worker playground.