Artifacts tool can now read and save multiple files in one call

What

• Tool-result rendering now recognizes a files_read result and shows "saved N of M file(s)", optionally "under " when a shared save directory is used; the WebFetch-style summary can also show "read N file(s)" for multiple paths.
• The artifacts tool's read_file action now accepts a paths array instead of only a single path, so several published files can be requested in one call and saved under a shared directory.
• The result formatter reports how many of N files were saved, lists each with size/content-type/sha256 (or an error), inlines full text for files under a size budget, and truncates the listing when the combined result would be too large.
• Path resolution for read_file was updated to handle the multi-path case using a different helper than the single-file path.
• Multi-path input is validated (duplicate paths are removed, a "too many paths" case is reported), but a batched read that needs permission is currently always denied, telling the model to read files one at a time first, since batch reads don't yet trigger their own permission prompt.

Why

This lets Claude read and save several files from an artifact in a single call instead of one at a time, though permission handling for batch reads hasn't caught up yet, so a batch request needing approval will currently fail until read singly.