Re-split library and preview when the editor is hidden; remember the editor's width

docs/architecture/10-interaction-and-feedback.md

@@ -181,6 +181,23 @@ lives in [04 · Routing & Global Events](04-routing-and-events.md).
   now has a home — the hidden-pane state landed in M6 with the toggle strip (below). The strip
   owns show/hide; wiring the splitter's Enter to it is an optional convenience, not required.
 
+**Editor-hidden layout.** Pane model: the two **side** panes carry stored widths; the **editor
+is the flex filler** (no stored width while shown). Show/hide is layout-aware only for the
+editor — hiding **captures** the width it had (`capturedEditorWidth`), showing **pins it back**
+and re-splits the rest at the side panes' ratio (`shownSideWidths`); side panes just flip
+visibility and flex-fill the freed space (spec §01A). So `togglePane` needs `panesInner`
+(panes-row width minus the toggle strip, measured by `PaneToggleStrip`) for the editor only.
+With the editor hidden, library and preview become adjacent and share a **second splitter**,
+`PaneSplitHandle` — separate from `ResizeHandle` because the geometry differs: `ResizeHandle`
+resizes one side pane while the editor absorbs the change (the opposite side is untouched);
+`PaneSplitHandle` re-proportions **two** panes with no filler between them (editor-hidden, the
+side panes render `flex: width 1 0` and share the span), the pure `splitLibraryWidth()` holding
+each above its min. **valuenow trade-off:** it reports the library's 0–100 position
+(`splitValue()`) from the **stored ratio**, deliberately container-independent — so unlike
+`ResizeHandle` it does **not** observe the container, and `aria-valuenow` can drift from the
+rendered position after a window enlargement (worst near the extremes). Drag/keyboard read live
+`clientWidth`, so resizing itself stays accurate; only the announced value drifts.
+
 _(Consulted via `/council` → WAI-ARIA APG `windowsplitter`. This bullet is the contract;
 cite it, not the APG file.)_