ui: the sidebar's tabs become one dock; Measure and Signals by keyboard section; session keeps the dock's state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -30,7 +30,10 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   not freeze a colour that is meant to follow the signal list), **the math channels with their
   graphs**, **the measurements pane**, and **`measure_stats`** — the measurements statistics
   toggle; written as a bare top-level boolean (`measure_stats = true/false`), defaults to `false`
-  when absent. Signal identity
+  when absent, and **`dock`** -- the Dock button's state (the user's last word on it, `sidebar_wanted`),
+  a bare top-level boolean written where the sidebar's `side_tab` used to be, default `true`. The parser
+  ignores unknown keys, so a file written before the tabs became a dock (`side_tab = "measure"`) still loads
+  and simply opens with the dock shown; nothing needs to read the old key. Signal identity
   is the *name* throughout (a CONFIG renumbers channels), 'enabled' is read off the channels the
   model built -- an enabled signal is exactly one that produced a channel -- and `apply_layout()`
   installs the graphs **before** assigning the rows, since a row may hold a math channel's name and