Take the sort of a large folder off the main thread

The 20000-file measurement had a shape worth following up: median 184 frames, but one iteration of five at 598. Reading the listing path found the reason it could be that shape. AllFilesSource.list already hops to Dispatchers.IO, but BrowserViewModel.recompute filtered and sorted whatever it returned on the caller's thread - and the default order is natural order, which parses the digits in every name on every comparison, so 20000 rows is a few hundred thousand string comparisons plus a fresh 20000-element list inside a tap.

recompute now sorts off the main thread above 500 rows and keeps the synchronous path below it, so the common case is untouched and the tests that read state immediately after an action still hold. The dispatcher is injected for exactly that reason, and the large path has its own test: 600 files, listed, in natural order. The state update is guarded on path, sort and hidden-files so a second tap cannot be overwritten by the answer to the first.

The fix is not re-measured: the 20000-file fixture takes half an hour to build through MIUI's storage layer, so PERFORMANCE.md keeps the 598 outlier as the motivation and says plainly that whoever has the fixture next should confirm the maximum came down.

docs/CODE_MAP.md

@@ -2,7 +2,7 @@
 
 Generated from the source by `tools/make-code-map.py` - do not edit by hand. Regenerate it after a change (`python3 tools/make-code-map.py`); `--check` exits non-zero when this file is stale.
 
-78 source files, 12856 lines - 34 test files, 390 tests. The behaviour index at the end lists every test name, and in this project a test name is a sentence about what the app does, so it doubles as the specification.
+78 source files, 12910 lines - 34 test files, 391 tests. The behaviour index at the end lists every test name, and in this project a test name is a sentence about what the app does, so it doubles as the specification.
 
 Start at `AGENTS.md`. What is done, and why: `docs/ROADMAP.md` and `CHANGELOG.md`. What was verified by hand rather than by a test: `docs/EDGE_CASE_TESTING.md`.
 
@@ -102,7 +102,7 @@ The browser screen: view model, state, list and grid, dialogs, selection bar
 | `BrowserScreen.kt` | 591 | BrowserScreen() | - |
 | `BrowserTopBar.kt` | 238 | The browser's app bar: title, up, the favourite star, and the search / sort / overflow actions. BrowserTopBar(), SEARCH_FIELD_WIDTH | `BrowserTopBarTest` (4) |
 | `BrowserUiState.kt` | 311 | One immutable description of everything the browser shows. ViewMode, SearchScope, Clipboard, UiMessage, ConflictPrompt, RootItem, Bookmark, DetailFocus, Undo, UndoMove, Overlay, BrowserUiState | - |
-| `BrowserViewModel.kt` | 2152 | Drives the browser screen. BrowserViewModel, ArchivePreview | `BrowserViewModelTest` (89) |
+| `BrowserViewModel.kt` | 2206 | Drives the browser screen. BrowserViewModel, ArchivePreview | `BrowserViewModelTest` (90) |
 | `DetailsSheet.kt` | 109 | Name, location, size, type and modified date for one entry. DetailsSheet(), DetailLines(), DetailLine() | - |
 | `EntryIntents.kt` | 58 | Wraps an entry's path in a content:// URI another app may read. contentUriFor(), viewIntent(), shareIntent() | - |
 | `IncomingBar.kt` | 60 | The bar shown while another app is waiting on this one. IncomingBar() | - |
@@ -572,13 +572,14 @@ Every test name in the suite, by file: the fastest way to see whether something
 - a library or the root list has nothing to star
 - the star gives way to the search field
 
-### `ui/browser/BrowserViewModelTest.kt` (89)
+### `ui/browser/BrowserViewModelTest.kt` (90)
 
 - starts on the root list
 - opening a folder lists it with directories first and natural order
 - hidden files are filtered out until asked for
 - search narrows the listing
 - sorting by size reorders the listing
+- a folder of six hundred files is still listed in natural order
 - navigating up goes to the parent then to the roots
 - selection survives toggling and clears on navigation
 - a file with no preview of its own is handed to the system