test(web): attachment font and large-text preview tests wait for the real load#26314

Two AttachmentFilePreview browser tests still failed on main after #25664 (4 main reds since 2026-10-26 16:29).

• retains the workspace viewer across URL renewals…: CI showed 64 lines · Indexing…, because the cold large-text worker had not posted its final meta count within vi.waitFor's 1s. The test's Worker stub now resolves a promise on that {type:"meta", complete:true} message, and the test awaits it before asserting 1,000 lines.
• loads a font attachment through the lazy preview: two chained lazy chunks (FontFilePreview → FontViewer) loaded cold inside one 1s wait. The test preloads both chunks, the same way the file already handles ReadOnlySourcePreview. The lazy render path is unchanged.

No timeouts, retries, or skips were added. Both tests passed repeated local runs, including with every core busy, and vp run check:changed passes. Neither failure reproduced locally on the original code either, so the causes come from the CI failure output.

Implemented with Claude Opus 5.5 in Claude Code.

🤖 Generated with Claude Code