Add Binding::Scroll as an axis input source

The mouse wheel's per-frame delta is a Vec2, so it maps directly onto
axis actions alongside gamepad sticks. hardware_to_actions now reads
AccumulatedMouseScroll in the Axis branch (max-magnitude combine); the
digital branch ignores it, and Scalar is unchanged.

Since an axis is refreshed every frame and the scroll delta is zero when
idle, on_action fires exactly while the wheel turns and the piped value
is the delta — no persistence or edge-guarding needed. Enables e.g.
`.bind(Action::Zoom, [Binding::Scroll])`.

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

engine/src/actions.rs

@@ -137,6 +137,7 @@ use core::marker::PhantomData;
 
 use bevy::{
     ecs::{schedule::ScheduleLabel, system::ScheduleSystem},
+    input::mouse::AccumulatedMouseScroll,
     platform::collections::{HashMap, HashSet},
     prelude::*,
     ui::Checked,
@@ -258,8 +259,8 @@ pub trait GameAction: Copy + Eq + Hash + Send + Sync + 'static {
 ///
 /// [`Key`](Self::Key), [`Mouse`](Self::Mouse) and [`Gamepad`](Self::Gamepad) are
 /// digital (for [`Instant`](ActionKind::Instant)/[`Toggle`](ActionKind::Toggle)
-/// actions); [`Stick`](Self::Stick) is analog (for [`Axis`](ActionKind::Axis)
-/// actions).
+/// actions); [`Stick`](Self::Stick) and [`Scroll`](Self::Scroll) are analog (for
+/// [`Axis`](ActionKind::Axis) actions).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Reflect)]
 pub enum Binding {
     /// A keyboard key.
@@ -271,6 +272,12 @@ pub enum Binding {
     /// A gamepad analog source producing a [`Vec2`] (matched across all connected
     /// gamepads; deadzone already applied by Bevy).
     Stick(Stick),
+    /// The mouse wheel, producing this frame's scroll delta as a [`Vec2`]
+    /// (`x` = horizontal, `y` = vertical). Like a stick, it drives
+    /// [`Axis`](ActionKind::Axis) actions — but it is a *delta* (non-zero only
+    /// while scrolling), so the axis reads zero on idle frames and
+    /// [`on_action`] fires exactly while the wheel turns.
+    Scroll,
 }
 
 /// An analog 2D source on a gamepad.
@@ -557,28 +564,39 @@ fn hardware_to_actions<T: GameAction>(
     keys: Res<ButtonInput<KeyCode>>,
     mouse: Res<ButtonInput<MouseButton>>,
     pads: Query<&Gamepad>,
+    scroll: Res<AccumulatedMouseScroll>,
     map: Res<InputMap<T>>,
     mut state: ResMut<ActionState<T>>,
 ) {
     for (&action, bindings) in &map.bindings {
         match action.kind() {
-            // Analog: pick the most-deflected reading across sticks/gamepads and
-            // store it every frame (zero when nothing is connected or centered).
+            // Analog: pick the strongest reading across analog sources and store it
+            // every frame (zero when nothing is deflected / not scrolling).
             ActionKind::Axis => {
                 let mut value = Vec2::ZERO;
                 for binding in bindings {
-                    let Binding::Stick(stick) = binding else {
-                        continue;
-                    };
-                    for pad in &pads {
-                        let reading = match stick {
-                            Stick::Left => pad.left_stick(),
-                            Stick::Right => pad.right_stick(),
-                            Stick::DPad => pad.dpad(),
-                        };
-                        if reading.length_squared() > value.length_squared() {
-                            value = reading;
+                    let reading = match binding {
+                        Binding::Stick(stick) => {
+                            // Best deflection across all connected gamepads.
+                            let mut best = Vec2::ZERO;
+                            for pad in &pads {
+                                let r = match stick {
+                                    Stick::Left => pad.left_stick(),
+                                    Stick::Right => pad.right_stick(),
+                                    Stick::DPad => pad.dpad(),
+                                };
+                                if r.length_squared() > best.length_squared() {
+                                    best = r;
+                                }
+                            }
+                            best
                         }
+                        // This frame's wheel delta (zero when not scrolling).
+                        Binding::Scroll => scroll.delta,
+                        _ => continue,
+                    };
+                    if reading.length_squared() > value.length_squared() {
+                        value = reading;
                     }
                 }
                 state.set_axis(action, value);
@@ -591,7 +609,8 @@ fn hardware_to_actions<T: GameAction>(
                         Binding::Key(k) => keys.just_pressed(*k),
                         Binding::Mouse(m) => mouse.just_pressed(*m),
                         Binding::Gamepad(g) => pads.iter().any(|pad| pad.just_pressed(*g)),
-                        Binding::Stick(_) => false, // not meaningful for digital actions
+                        // Analog sources are not meaningful for digital actions.
+                        Binding::Stick(_) | Binding::Scroll => false,
                     };
                     if just_pressed {
                         match kind {