view: a library with a required param can be opened but not edited #2065

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Problem

A library module with a required param can be opened in the view, but no edit to it is accepted.

• Loading uses viewOf in web/view/src/engine.ts, which falls back to the structure alone when the module cannot be evaluated.
• Editing (GitHubHost.edit in web/view/src/githubHost.ts) requires evaluateView to succeed.

So after opening or pinning such a library, every valid edit is rolled back with required param x has no value.

Proposed fix

• Merge viewOf and evaluateView.
• Treat "statically valid", "can be given values" and "evaluation failed" as separate states, and accept an edit that leaves the module statically valid.