feat(viewport): RMB 1st=Enter, 2nd=menu (no setting)

Replaces the opt-in `rmb_enter` setting (and `RMBENTER` command) with
always-on behavior. While a command is active, the first viewport
right-click acts as Enter (commit / close) and a second consecutive
right-click opens the context menu instead. Idle right-click still opens
the menu; right-drag still orbits.

The cycle is tracked per tab (`right_click_entered`) and restarts on any
other interaction — a left-click pick, a new command, an orbit, or
leaving the viewport — so the next right-click is Enter again.

Refines #185.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pick/selection_state.rs

@@ -27,6 +27,11 @@ pub struct SelectionState {
     pub right_press_time: Option<Instant>,
     pub right_dragging: bool,
     pub right_last_pos: Option<Point>,
+    /// While a command is active, a right-click acts as Enter; the *next*
+    /// consecutive right-click opens the context menu instead. This tracks
+    /// whether the previous right-click already fired Enter. Reset by any
+    /// other interaction (left-click pick, a new command) and on viewport exit.
+    pub right_click_entered: bool,
     pub middle_down: bool,
     pub middle_last_pos: Option<Point>,
     pub middle_last_press_time: Option<Instant>,