fix: load designs off-thread and cache recent thumbnails as PNGs

docs/control-protocol.md

@@ -46,6 +46,7 @@ Commands never open dialogs when run this way. `solvecraft-cli commands` prints
 | `ui.render` | `path`, `width?`, `height?` | CPU render of the model with the current camera (no window needed) |
 | `ui.confirm` | `accept?`: bool | answers the Delete confirmation (shown when a delete takes other features with it or makes some fail); without `accept`, what it lists |
 | `ui.documents` | `action?`: `new`, `switch`, `close`, `quit`; `index?`; `force?` (close without asking); `answer?`: `save`, `dont_save`, `cancel` (answers "save changes?") | the open designs (tabs): names, unsaved flags, the active one, a pending "save changes?" prompt, whether a quit is asking |
+| `ui.file` | `path`; `insert?`: bool | starts an interactive background Open or Insert; poll `ui.inspect.file_busy` for completion; the window stays responsive |
 | `ui.home` | `open?`: bool; `sample?`: index | the start page: show or hide it, open a built-in sample (once built); returns `open` and the recent designs |
 | `ui.shortcut` | `command`, `key?`, `replace?` | a command's key, or rebinds it (`""` clears; a key another command uses is refused unless `replace`) |
 | `ui.help` | `item?`: `about`, `shortcuts` (read only), `report` (copies diagnostics) | the version, commit, build date and diagnostic text |

docs/perf.md

@@ -113,3 +113,43 @@ errors) through edits, suppression, roll-back, undo and redo. Setting a sketch d
 can still recompute (oracle 61, 62): solving the sketch moves its points by rounding noise, so
 its inputs are not exactly the old ones. Oracle 59's first parameter edit fails (a recipe
 expression the bench's `* 1.02` can't scale) and is left out.
+
+
+## Desktop loading and recent thumbnails (2026-10-10)
+
+The Arduino Uno enclosure (19 features, 34 KB) and Raspberry Pi 5 case (24 features,
+61 KB) each decode in about 1 ms. A release build spends roughly 12.1 s and 8.2 s,
+respectively, evaluating their geometry on this Windows machine. Changing the document
+encoding does not eliminate these boolean operations.
+
+Interactive Open, recent-file clicks, drag/drop and STEP/mesh insertion now execute on a
+worker, including preparation of the world-space bodies and display meshes. The existing
+design stays available until the result is ready; a failed or cancelled job cannot replace
+it. Insertion preserves the file, saved state, earlier undo history and view settings.
+Timeline copies share their lazy mesh cell, and adopting a prepared session keeps its warm
+world-space cache instead of transforming and tessellating its components again on the UI.
+A STEP body's missing-face check reuses its orientation tessellation when the tolerances
+match and its orientation was not reversed.
+
+Live desktop tests (`ci` profile, with other builds/tests running) opened the Arduino and
+Pi case in 15.2 s and 10.5 s. During those jobs, median `ui.inspect` round trips were 20.5 ms
+and 8.3 ms; the slowest were 33.7 ms and 28.6 ms. This fixes the window freeze, not the
+remaining cost of cold geometry evaluation.
+
+Recent thumbnails are 240 × 150 PNG files in `SolveCraft/thumbnails` under the platform
+cache directory (`LOCALAPPDATA` on Windows). Their keys include the source path, file size
+and modification time. Open and Save render from the model already in memory. Cached
+recents decode the PNG without reading or rebuilding a document, and run before cold
+recents and sample thumbnails. Missing or invalid pictures fall back to a background rebuild.
+No thumbnail or base64 image is added to the design JSON. A regression test verifies the
+source bytes remain unchanged, a cached picture works even without a parseable source
+file, and changing the source invalidates the cache.
+
+`solvecraft-cli bench-open` also accepts STEP/IGES. For exchange files its `parse` phase
+includes constructing and checking the imported B-rep, not merely decoding STEP syntax.
+
+The live test loaded both cached recent cards in 21 ms and inserted the 2 MB lightweight
+Pi STEP (27 bodies) into the case in 279 ms; undo restored the original two bodies. The
+full 77 MB board STEP (2689 solid definitions, 45,689 faces) still had not completed after
+300 s. Its window continued handling inspection and Cancel, and cancelling preserved the
+case. Complex STEP construction remains an outstanding kernel performance problem.