session: remember extra windows, and reopen them from the menu

A scope or detached window was never written to the session, and a detached figure vanished from
the layout at the next start -- the figure had been removed from the main rows, and nothing kept
the window that held it.

Each window is now a `[[window]]` block in the session -- `name`, `detached`, and its own
`[[window.plot]]` rows with the same keys the main `[[plot]]` rows carry (signals, row weight,
cell width). `collect_layout()` writes the open windows' present arrangement plus the ones already
closed; loading puts **all** of them in `App::closed_windows` and never opens one: a window is a
view the user closed, and a launch that tiled windows onto the desktop would take that decision
for them (a tiling WM would tile them straight away). The overflow menu gains **Reopen closed
windows (N)**, which opens every remembered window through `open_scope_window_from_state()` -- the
one window constructor, then its rows, weights and signals dealt into its row set.

Closing a window records it, by name (a close/reopen cycle replaces its entry rather than
multiplying them), and schedules the save; **attaching** records nothing, because that window's
figure is on the main window again. A detached figure therefore survives a quit through its
window's entry.

Probe: `[scope-probe] loaded closed=N scopes=0` (remembered, none opened) and `reopen
scopes=..->.. remembered=.. left=0` (close, remember, the menu item's own click reopens).
Docs: README, the Mallard layout page, the probe readings and the session invariant.

docs/claude/invariants-session-and-export.md

@@ -204,3 +204,14 @@ one, which is the only ordering in which a switch cannot lose what it switches f
   configuration's folder): an upgrade keeps the events it had. The key is a *sanitized* path
   component, the same rule the configuration files use, so a name typed with a slash cannot
   leave the directory.
+
+- **The session remembers extra windows; a launch never opens them.** Each scope or detached
+  window is a `[[window]]` block (`name`, `detached`, and its own `[[window.plot]]` rows -- the
+  same keys the main `[[plot]]` rows carry), written from the open windows' present arrangement
+  plus the ones already closed (`collect_layout()`). Loading puts **all** of them in
+  `App::closed_windows`, never into `app.scopes`: the overflow menu's *Reopen closed windows (N)*
+  is the only path that opens them (`open_scope_window_from_state()`, the one constructor every
+  window uses, then its rows). Closing a window records it (by name, replaced, so close/reopen
+  cycles do not multiply entries); **attaching** records nothing -- its figure is on the main
+  window again -- and a detached figure survives a quit through its window's entry instead of
+  being lost from the main layout.