Add resizable panes with drag handles, min widths, and persistence

docs/architecture/02-persistence.md

@@ -18,7 +18,7 @@ src/
 │       ├── snippet-store.ts     # IndexedDB: snippets (metadata + drafts)
 │       ├── dataset-store.ts     # IndexedDB: datasets (heavy payloads)
 │       ├── settings-store.ts    # localStorage: UserSettings
-│       └── prefs-store.ts       # localStorage: app/UI prefs (sort, layout)
+│       └── ux-prefs.ts          # localStorage: app/UI prefs (sort, panel layout)
 ```
 
 ### Why this boundary exists
@@ -317,17 +317,24 @@ export function saveSettings(s: UserSettings): void {
 }
 ```
 
-App/UI prefs follow the identical pattern under their own keys, e.g.:
+App/UI prefs follow the identical guard+fallback pattern, but live in **one
+record under their own key**, `astrolabe:ux-prefs` (the snippet sort and the
+panel layout together — the plan's "ux-prefs for sort + panel layout", §09D):
 
 ```ts
-// src/app/infrastructure/prefs-store.ts
-const SORT_KEY = 'astrolabe:snippet-sort';
-const LAYOUT_KEY = 'astrolabe:panel-layout';
+// src/app/infrastructure/ux-prefs.ts
+const KEY = 'astrolabe:ux-prefs'; // { panelLayout: { libraryWidth, previewWidth }, sort: { … } }
 
-const SORT_DEFAULTS = { sortBy: 'modified' as const, sortOrder: 'desc' as const };
-// loadSort()/saveSort() and loadLayout()/saveLayout() mirror §5's guard+fallback shape.
+// loadPanelLayout()/savePanelLayout() (and sort later) mirror §5's guard+fallback shape,
+// merging the changed section so a frequent write (a drag) never clobbers the others.
 ```
 
+> One nuance vs. the settings record: the panel-layout widths are **validated**
+> (positive finite numbers) and surfaced as `undefined` when absent/invalid; the
+> **defaults live in the `PanesStore`** (`PANE_DEFAULT`), applied at `hydrate`,
+> rather than merged in the adapter. Persistence of the live drag is **debounced**
+> in `orchestration/panes.ts` (a drag emits an update per pointer move).
+
 > **Do:** keep a single complete `DEFAULTS` object as the source of truth and merge over it.
 > **Don't:** read individual keys with bespoke `?? fallback` at each call site; one stale default and the shapes drift.