Add pane show/hide toggle strip with persisted visibility (M6, §01A)

A persistent left-rail toolbar (APG toolbar + roving tabindex) toggles the
library / editor / preview panes; a hidden pane frees its space and the rest
redistribute proportionally to their remembered widths. Visibility persists to
astrolabe:ux-prefs (separate key from widths, so a hidden pane keeps its width).
Adds the positional pane-* glyph sub-family to Icon and a Datasets shortcut
button in the strip.

src/app/stores/PanesStore.test.ts

@@ -93,3 +93,48 @@ describe('usePanesStore', () => {
     expect(store().previewWidth).toBe(PANE_DEFAULT.preview);
   });
 });
+
+describe('pane visibility (§01A)', () => {
+  test('all panes are visible by default', () => {
+    store().hydrate({}, undefined);
+    expect(store().libraryVisible).toBe(true);
+    expect(store().editorVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+  });
+
+  test('togglePane flips only the targeted pane', () => {
+    store().hydrate({}, {});
+    store().togglePane('editor');
+    expect(store().editorVisible).toBe(false);
+    expect(store().libraryVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+
+    store().togglePane('editor');
+    expect(store().editorVisible).toBe(true);
+  });
+
+  test('hiding all panes is permitted', () => {
+    store().hydrate({}, {});
+    store().togglePane('library');
+    store().togglePane('editor');
+    store().togglePane('preview');
+    expect(store().libraryVisible).toBe(false);
+    expect(store().editorVisible).toBe(false);
+    expect(store().previewVisible).toBe(false);
+  });
+
+  test('hydrate restores explicit visibility; missing flags default to shown', () => {
+    store().hydrate({}, { library: false });
+    expect(store().libraryVisible).toBe(false);
+    expect(store().editorVisible).toBe(true);
+    expect(store().previewVisible).toBe(true);
+  });
+
+  test('a hidden pane keeps its remembered width (re-shows at it)', () => {
+    store().hydrate({ libraryWidth: 250 }, {});
+    store().togglePane('library'); // hide
+    expect(store().libraryWidth).toBe(250);
+    store().togglePane('library'); // re-show
+    expect(store().libraryWidth).toBe(250);
+  });
+});

src/app/stores/PanesStore.ts

@@ -22,6 +22,9 @@ import { create } from 'zustand';
 /** Which side pane a drag handle controls. */
 export type PaneSide = 'library' | 'preview';
 
+/** Every pane that can be shown/hidden via the toggle strip (spec §01A). */
+export type PaneName = 'library' | 'editor' | 'preview';
+
 /** Minimum widths (px) enforced while resizing, so no pane collapses (§01A). */
 export const PANE_MIN = { library: 180, preview: 240, editor: 320 } as const;
 
@@ -79,25 +82,56 @@ export function sideWidthValue(
   return Math.round(Math.min(100, Math.max(0, pct)));
 }
 
+/** Per-pane visibility (spec §01A). Widths are kept *independently* of visibility,
+ * so a hidden pane keeps its remembered width and re-shows at it (not a default). */
+export interface PaneVisibility {
+  library?: boolean;
+  editor?: boolean;
+  preview?: boolean;
+}
+
 export interface PanesState {
   libraryWidth: number;
   previewWidth: number;
+  /** Whether each pane is currently shown. All visible by default (§01A). */
+  libraryVisible: boolean;
+  editorVisible: boolean;
+  previewVisible: boolean;
   /** Set a side pane's width (already clamped by the caller). */
   setWidth: (side: PaneSide, width: number) => void;
-  /** Restore persisted widths on startup; missing values keep their defaults. */
-  hydrate: (layout: { libraryWidth?: number; previewWidth?: number }) => void;
+  /** Show/hide one pane. Hiding all panes is permitted (§01A) — the strip stays. */
+  togglePane: (pane: PaneName) => void;
+  /** Restore persisted widths + visibility on startup; missing values keep defaults. */
+  hydrate: (
+    layout: { libraryWidth?: number; previewWidth?: number },
+    visibility?: PaneVisibility,
+  ) => void;
 }
 
+const VISIBLE_KEY = {
+  library: 'libraryVisible',
+  editor: 'editorVisible',
+  preview: 'previewVisible',
+} as const;
+
 export const usePanesStore = create<PanesState>((set) => ({
   libraryWidth: PANE_DEFAULT.library,
   previewWidth: PANE_DEFAULT.preview,
+  libraryVisible: true,
+  editorVisible: true,
+  previewVisible: true,
 
   setWidth: (side, width) =>
     set(side === 'library' ? { libraryWidth: width } : { previewWidth: width }),
 
-  hydrate: (layout) =>
+  togglePane: (pane) => set((s) => ({ [VISIBLE_KEY[pane]]: !s[VISIBLE_KEY[pane]] })),
+
+  hydrate: (layout, visibility) =>
     set({
       libraryWidth: layout.libraryWidth ?? PANE_DEFAULT.library,
       previewWidth: layout.previewWidth ?? PANE_DEFAULT.preview,
+      libraryVisible: visibility?.library ?? true,
+      editorVisible: visibility?.editor ?? true,
+      previewVisible: visibility?.preview ?? true,
     }),
 }));