view: deduplicate logic in web/view #2081

Found in the review of #2057, which adds the experimental view. Paths refer to #2059's head.

Summary

Logic written more than once in web/view/src/. Only copies that do the same job are listed:

• viewOf and evaluateView in engine.ts; merging them is part of view: a library with a required param can be opened but not edited #2065.
• Guessing a file name from a module id, in render.ts and main.ts.
• Joining paths under the project root: three places in githubHost.ts, and repoProject.join.
• GitHubHost.readBase and GitHubHost.load.
• Small helpers: el(), errorMessage, SVG_NS, wire end points, lane y coordinates (wires.ts, cross.ts), clamping the scale, hasSub.
• minimalEdit lives in vscodeHost.ts but editor.ts uses it too; move it to source.ts.