Merge pull request #519 from aakhter/pr/grouped-rail-tree

feat(tabs): full-row activation and tree semantics for the grouped rail

docs/architecture-invariants.md

@@ -303,6 +303,8 @@ So: `_confirmIdle()` (session.ts) requires the pane to go quiet, and then asks t
 - **Collapse is per-device** (`codeman:tab-groups-collapsed` in localStorage, ids of deleted groups garbage-collected on adoption, any storage failure means all-expanded). A collapsed group still SHOWS the active row, and `_updateActiveTabImmediate` falls through to a full render whenever the structure key changes, since a class toggle cannot reveal a hidden row.
 - **Lineage arcs to a collapse-hidden session anchor to its group header** (`lineage-line--proxied`); two endpoints proxied to one header draw nothing.
 - **Drag-reorder is off in the grouped rail** until grouped editing lands: a flat-order drop cannot express a group move, and the server re-ranks within the old group.
+- **Only the grouped rail is a tree.** `#sessionTabs` ships as `role=tablist` with `role=tab` rows, and the header strip, sidebar and flat rail keep exactly that. While grouped, `_applyTabListRole` makes it `role=tree` (and restores `tablist` + its label when grouping ends), named-group headers are level-1 `treeitem`s that `aria-owns` their rows' `role=group` (rows sit beside the header, not inside it), and ungrouped rows plus a collapsed group's kept selection are level-1 items. A collapsed header owns nothing, and the "Ungrouped" heading is `aria-hidden`. Rows are re-roled in the DOM by `_applyTabTreeSemantics` after render, never by rewriting their markup, so a grouped row's content stays the flat row's.
+- **One tab stop in the tree.** Exactly one treeitem carries `tabindex=0` (the focused or selected item); every control inside a row drops to `-1`, which is why Shift+F10 / ContextMenu open a row's actions from the keyboard. Focus survives a full re-render by identity (`group:`/`session:`/`webview:`; a row a collapse just hid hands focus to its header), but only when focus was already inside the rail. The tree walk (`_tabTreeItems`) follows painted order WITHIN each group when the rail is sorted; the flat list keeps its own whole-list computed-order walk.
 
 
 ⚠️ **`TabLayoutService` is the single mutation boundary** and every lifecycle caller (session created/removed, webview created/deleted, a legacy order PUT) describes ONE completed server action and gets AT MOST ONE versioned write; writing layout state from a route or a manager directly is what the service exists to prevent.