Adds URL-backed artifact rendering for local and HTTPS files through ArtifactCanvas(url:).
- Resolves MIME types from explicit declarations, magic bytes, filenames, HTTP metadata, and Uniform Type Identifiers.
- Routes text and resolved local file URLs through the existing environment renderer registry, including structured JSON and CSV suffix fallbacks.
- Adds bounded downloads and text decoding, HTTPS-first policy, typed errors, retry handling, isolated caches, and security-scoped file support.
- Moves PDF and raster file reads off the main actor and adds generic text and binary fallbacks.
- Adds deterministic resolver, cache, MIME, renderer contract, and integration coverage.