add concept of "quick switch to last selected security" to todo

TODO.md

@@ -42,6 +42,14 @@ Commands:
 - `src/commands/quote.zig`
 - `src/commands/splits.zig`
 
+## TUI: toggle to last symbol keybind
+
+Add a single-key toggle that flips between the current symbol and the
+previously selected one (like `cd -` in bash or `Ctrl+^` in vim). Store
+`last_symbol` on `App`; on symbol change, stash the previous. The toggle
+key swaps current and last. Works on any tab — particularly useful for
+eyeball-comparing performance/risk data between two symbols.
+
 ## Market-aware cache TTL for daily candles
 
 Daily candle TTL is currently 23h45m, but candle data only becomes meaningful