View menu: tree/palette toggle, and Back/Forward through opened sheets

Tree / Palette hides the left column so the wiresheet has the whole window.
ttk has no hide flag for a pane, so the column is dropped from the paned
window and re-inserted at the front, which is why the configured width is
re-applied afterwards.

Back and Forward walk the sheets opened this session, with two arrow buttons
in a new toolbar on the same commands. History entries hold the element whose
children are painted rather than a tree item id: rebuilding the tree hands out
fresh ids, while the elements themselves survive. Recording runs from the tree
selection handler, the one route every sheet opening takes, and is suppressed
while a walk drives that selection so it cannot rewrite its own trail. Opening
a different file retires the history; a delete or an undo drops only the
entries whose element has left the document, keeping the position on screen.

open_file builds the tree inline instead of going through
refresh_treeview_from_dom, so it never raised the tree-filled callback and the
history had no way to see a new file. It and reset_to_startup_state now fire
it, which also gets the tree column fitted on load rather than on the first
resize.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -82,10 +82,27 @@ Undoing back to the depth of the last save clears the dirty tint rather than lea
 | `Ctrl`/`Cmd` + `Z` | Undo |
 | `Ctrl`/`Cmd` + `Y` or `Ctrl`/`Cmd` + `Shift` + `Z` | Redo |
 | `Del` / `Backspace` | Delete selected component |
+| `Ctrl`/`Cmd` + `B` | Show or hide the tree/palette column |
+| `Alt` + `Left` / `Alt` + `Right` | Back and forward through the sheets opened this session |
 
 Both `Command-` and `Control-` are bound for each, so the same build works on macOS and
 Windows; menu accelerators render as `Cmd` or `Ctrl` per platform.
 
+### The View menu
+
+**Tree / Palette** hides the left column so the wiresheet has the whole window, and puts
+it back at its configured width. ttk has no hide flag for a pane, so the column is dropped
+from the paned window and re-inserted at the front.
+
+**Back** and **Forward** walk the sheets opened this session, mirrored by the two arrow
+buttons in the toolbar. The history records the element whose children are painted -- the
+same thing `app_state.current_sheet_parent` holds -- rather than a tree item id, because
+rebuilding the tree hands out fresh ids while the elements themselves survive. Opening a
+different file retires the history; a delete or an undo drops only the entries whose
+element has left the document, keeping the position on screen. Walking the history drives
+the tree selection, which is what paints the sheet, and is suppressed from recording so a
+walk cannot rewrite the trail it is following.
+
 ### Testing without a window
 
 `app_state`, `sax_dom`, `schema`, `undo` and `wiresheet` import with no display, so the