Retire the legacy menu mirror and make MenuDocument the single authored model

.prettierignore

@@ -1,5 +1,6 @@
 .claude
 dist
+dist-js
 node_modules
 coverage
 .pnpm-store

SPEC.md

@@ -690,6 +690,17 @@ That means generated menus should derive from:
 
 Generated menus should remain editable after creation. The user should be able to accept a generated baseline and then customise layout, navigation, labels, and art.
 
+### 14.5 Generated menu navigation
+
+Autogenerated menus (currently chapter-selection grids, with the same generator machinery available to other generated menu types) ship with fully wired remote-navigation, not placeholder buttons the user must connect manually.
+
+- Every generated button gets up/down/left/right links from its layout geometry — a grid wires row/column neighbours, a single-column list wires a simple vertical chain.
+- Every link the generator creates is bidirectional: linking a button's `navRight` to a neighbour also sets that neighbour's `navLeft` back to it, and so on for the other three directions. This guarantees no generated button is reachable only in one direction and unreachable in the other, which would strand it behind a remote press with no way back.
+- A trailing utility row (Previous/Back/Next, for paginated chapter grids) is chained left-to-right by position and anchored onto the closest column of the grid's last row, so the utility row and the grid form a single connected navigation graph rather than two disjoint ones.
+- Each generated menu sets `interaction.defaultFocusId` to a real button (the first grid item, falling back to the first utility button for pages with no grid items) so the menu has a sane initial remote-navigation focus the moment it's generated, before any manual editing.
+
+Generated menus remain fully editable after creation — a user can rewire, add, or remove any of these links from the authored navigation model like any hand-authored menu.
+
 ### 14.7 Menu themes and style architecture
 
 Menu themes do not need to be implemented in v1, but the architecture should preserve room for them early.