Keep the UI-zoom tests: set the interface scale with ui.zoomFactor now that Cmd+Plus zooms the photo (#566)

The CLI snapshot test and the headless resize test sent Cmd+Plus to change egui's
interface scale; with the keys handed to View > Zoom they no longer did. A
ui.zoomFactor control request (0.5-3, documented) sets the scale explicitly.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -42,6 +42,7 @@ mode ([mcp.md](mcp.md)) is a thin layer over this channel. Implementation:
 | `ui.set` | partial UI state, e.g. `{"view": "detail"}` | Resulting UI state |
 | `ui.dialog.confirm` / `ui.dialog.cancel` | — | Close the open dialog |
 | `ui.resize` | `{width, height}` | Resize the window |
+| `ui.zoomFactor` | `{factor}` | Set the interface scale (0.5–3; egui's zoom keys are off, Cmd+= / Cmd+- zoom the photo) |
 | `ui.screenshot` | `{path?, headless?}` | `{path, width, height}` once the frame (with finished renders) is captured. `headless: true` draws the UI on the CPU (no compositor needed); a windowed capture that gets no frame within 2 s falls back to headless automatically |
 | `engine.execute {command: "app.export", params}` | export params (see `docs/mcp.md`), plus `preset`, `dir` / `path`, `ids`, `background` | Writes the files and returns `{files}`; with `background: true` (what the Export dialog and menus use) it returns `{background: true, total}` at once and the batch runs on a worker thread — poll `ui.inspect` → `export` |
 | `ui.render` | `{id?, size?, path?}` | Render a photo (PNG to `path`), `{width, height}` |