Copy published-subset files into ro-crate-preview-files/ during build

[benfoley]

Summary

• "Publish subset only" (ro-crate-html-output's publishOnly option) already filters the metadata graph to just the published collections/objects/files (filterCrateToPublished), but never touched the actual files on disk — a folder holding just the generated preview still exposed every file, published or not.
• New collectFilePaths(crate) collects the folder-relative @ids of every File entity that survives the filter (skipping externally-hosted files, same guard reconcileFileEntities uses in chaos2crate's crate.js).
• New copyPublishedFilesToPreviewFolder(crate, dirHandle, log) copies each of those files into a fresh ro-crate-preview-files/ directory (wiping a stale one from a previous build first, same pattern as ro-crate-preview_html/), warning (not failing) on any file the crate references but the folder no longer has.
• New rewriteToPreviewFilesFolder(html, assetMap) redirects every href="…"/src="…" and CSS url(…) reference to one of those files, from its normal crate-relative path to its copy — applied to the root ro-crate-preview.html and to every multipage file under ro-crate-preview_html/. It matches each known file path as one literal string rather than splitting at the first #/?, so a file whose own name contains one of those characters (real crates have these) still resolves correctly instead of being mistaken for a URL fragment/query.
• ro-crate-preview-files is declared in the plugin's outputPaths, so it's automatically excluded from the next build's directory scan and cleaned up by the existing "delete plugin output before rebuilding" setting, the same as ro-crate-preview_html.
• Deliberately scoped to the generated HTML only — ro-crate-metadata.json/.xlsx are unaffected, matching how publishOnly already worked before this change.

Test plan

• New tests in chaos2crate's tests/test-publish-preview-files.mjs (this repo has no local test runner of its own — the sibling app's suite exercises it via the file:../c2c-plugins dependency) cover collectFilePaths, copyPublishedFilesToPreviewFolder (with faked FSA deps), and rewriteToPreviewFilesFolder, including a filename containing #.
• chaos2crate's full npm test passes (one pre-existing, unrelated failure in test-default-profile.mjs that reproduces on its main branch too)
• chaos2crate's npm run build passes against this branch (via the local file:../c2c-plugins link)

Closes #27

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