T20 round 1 fixes

- the hex demo's selection stops at memory's end
- a disassembly gutter fills its row, so an empty one takes a press
- a breakpoint's box keeps its press from the row
- ruler ticks keep the places their step needs
- the toolbar names each usual key for the platform; the app binds them
- tests: gutter press, box press, tick places

examples/gallery/pages/debug/machine.rs

@@ -141,7 +141,7 @@ pub fn machine(window: &mut Window, cx: &mut App) -> impl IntoElement + use<> {
         frame().h(px(220.)).child(
             HexViewer::new("debug-hex", memory())
                 .base(0x7ffd_5a20_1000)
-                .selected(now_picked..now_picked + 4)
+                .selected(now_picked..(now_picked + 4).min(memory().len()))
                 .on_select(move |at, _, cx| set(&pick, at, cx)),
         ),
     )

examples/gallery/pages/debug/session.rs

@@ -106,7 +106,7 @@ pub fn session(window: &mut Window, cx: &mut App) -> impl IntoElement + use<> {
     let (command, tell) = (state.clone(), said.clone());
     section(
         "DebugToolbar",
-        "The floating row that drives a session: continue or pause, the three steps while paused, restart and stop, each with its key.",
+        "The floating row that drives a session: continue or pause, the three steps while paused, restart and stop. Each tooltip names the usual key; the app binds it.",
         cx,
     )
     .child(row().child(DebugToolbar::new("debug-toolbar", now_state).on_command(move |which, _, cx| {