LR-VIEW-COMPARE: Compare and Reference panes say "Rendering…" as the loupe does

Compare and Reference View are where focus is checked at 1:1, and their panes show the same
stand-ins as the loupe: a thumbnail while the photo renders, the last render after an
edit, the magnified frame under a window not in yet. Each pane now shows the loupe's
"Rendering…" pill once that has lasted half a second with nothing new (widget
`compare:rendering:<pane>`), and its failed pill ("This photo can't be opened", or
"Original offline · no smart preview", the reason on hover) when the render failed behind
a thumbnail (`compare:renderFailed:<pane>`). Survey's small fit tiles don't.

Compare panes have no draft renders, so a slider drag on the photo would leave its pane
saying "Rendering…" for the whole drag: the photo being dragged doesn't count until the
drag ends. Each pane's wait belongs to the photo in it, so one that moves in (swap, Make
Select) starts its own. A retry of a failed render keeps the failed pill, as in the loupe.

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

docs/control-protocol.md

@@ -73,7 +73,7 @@ filmstrip is visible.
 | `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. While the loupe still waits for a render, `ui.widgets {filter: "loupe:rendering"}` lists its "Rendering…" pill; `loupe:renderFailed` is the pill saying a render failed behind a stand-in |
+| `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. While the loupe still waits for a render, `ui.widgets {filter: "loupe:rendering"}` lists its "Rendering…" pill; `loupe:renderFailed` is the pill saying a render failed behind a stand-in; Compare and Reference panes have their own (`compare:rendering:<pane>`, `compare:renderFailed:<pane>`) |
 | `engine.execute {command: "app.export", params}` | export params (see `docs/mcp.md`), plus `preset`, `dir` / `path`, `ids`, `background` | Writes the files and returns `{files}` (one `{path, width, height, bytes, sidecars}` per file, plus `gpuFallback` — why that render fell back from the GPU to the CPU — when it did; see `docs/gpu-pipeline.md`); 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` |
 | `engine.execute {command: "activity.list"}` | — | The long-running tasks in flight (imports, exports, preview builds, downloads, the face scan…), oldest first: `{tasks: [{id, kind, label, done, total, unit, detail, cancellable, cancelling, ageMs}]}`; `total` 0 = not known yet; `unit` is `count`, `bytes` or `percent`; `label` is English (the app shows it translated) |
 | `engine.execute {command: "activity.cancel"}` | `{id}` or `{all: true}` | Asks the task (or every cancellable one) to stop at its next file or photo → `{cancelled: n}`; an unknown id (it may have just finished) or a task that can't be cancelled is an error |