view: browser smoke test, model contract test and unit tests

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

Summary

Browser smoke test. There is no end-to-end test of the view; site-verify.mjs checks only that the files exist and that the CSP meta tag is present. Add a smoke spec with api.github.com mocked: render a sheet, edit a cell, open and pin another file, include a module with different bindings, and round-trip a patch.
Model contract test. Nothing checks that the JSON Rust writes passes the page's zod schemas. Add a test that feeds graphcal view output for the fixtures to the schemas.
Pure modules without tests: layout, wires, rows, selection, panes, formula, literalFor, mapLimit and TokenStore. (view: fix the must-fix findings of the #2057 review #2059 added tests for diff, source, values, cross and modelCache.)
Bundle size. The view's JavaScript bundle has no size budget.

When fixing a view issue, first turn its reproduction into a failing test.