Add galaxy parameter UI, star selection, and movement/physics scaffolding

Galaxy creation scene (Bevy 0.16):
- Split into module folder: params.rs, mod.rs (generation + spawn), ui.rs,
  selection.rs
- GalaxyParams resource (seed, count, arms, vertical_arms, size, twist,
  vertical_twist) with generation counter for change detection
- Control panel: 7 +/- slider rows + Regenerate button, no native Bevy slider
  widget so uses label + icon buttons
- Info panel: live-refreshes on selection change via despawn_related::<Children>
- Click-to-select: screen-space picking (project each star to viewport, closest
  within 18px threshold wins); selected star lerps scale to 2.2x
- Generation faithfully ports the docs TS reference including vertical arms
- Regeneration system: despawns GalaxyScene root only, preserves UI panels

Camera:
- Camera2d -> Camera3d + OrbitCamera (left-drag yaw/pitch, scroll zoom,
  pitch/distance clamped to avoid gimbal)
- Orbit drag suppressed when cursor over UI

New plugins (scaffolding):
- movement/: Velocity, MaxSpeed, TurnRate, Drag, MoveTarget, Player components
  + click-to-move (no player spawned yet)
- physics/: pure-data geometry (ray_vs_sphere, overlaps, separate,
  segment_vs_sphere) with 7 passing unit tests; no systems wired yet
- star_map/: plugin skeleton gated on AppState::InGame

Shared:
- ui/util.rs: cursor_over_ui helper for UI-hover detection
- docs: ARCH-9 decision record (custom movement & collision, no physics engine)

apps/game/src/gameplay/galaxy_creation/selection.rs

@@ -0,0 +1,212 @@
+//! Click-to-select star systems + selection visuals + info panel refresh.
+//!
+//! Picking is done in **screen space**: each star's world position is projected
+//! to viewport coordinates, and we find the closest one within a pixel
+//! threshold. This is cheaper and more intuitive than raycasting against
+//! individual sphere meshes for an unbounded number of tiny stars.
+
+use bevy::prelude::*;
+use bevy::window::PrimaryWindow;
+
+use super::StarSystem;
+use crate::camera::MainCamera;
+use crate::gameplay::galaxy_creation::params::SelectedStar;
+use crate::gameplay::galaxy_creation::ui::GalaxyInfoPanel;
+use crate::ui::util::cursor_over_ui;
+
+// ── Tunables ────────────────────────────────────────────────────────────────
+
+/// Cursor-to-star screen distance (in logical pixels) below which a click is
+/// treated as a hit. The docs demo uses similar ~16px tolerance.
+const SELECTION_PIXEL_THRESHOLD: f32 = 18.0;
+
+/// Target visual scale for the selected star; non-selected stars animate back to 1.0.
+const SELECTED_SCALE: f32 = 2.2;
+/// Lerp speed (per second) for the scale animation. Higher = snappier.
+const SELECTION_LERP_SPEED: f32 = 10.0;
+
+// ── Pick on click ───────────────────────────────────────────────────────────
+
+pub fn select_star_on_click(
+    mouse_input: Res<ButtonInput<MouseButton>>,
+    primary_window: Query<&Window, With<PrimaryWindow>>,
+    camera_query: Query<(&Camera, &GlobalTransform), With<MainCamera>>,
+    stars: Query<(Entity, &Transform), With<StarSystem>>,
+    ui_nodes: Query<(&bevy::ui::ComputedNode, &GlobalTransform)>,
+    mut selected: ResMut<SelectedStar>,
+) {
+    if !mouse_input.just_pressed(MouseButton::Left) {
+        return;
+    }
+    let Ok(window) = primary_window.single() else {
+        return;
+    };
+    if cursor_over_ui(window, &ui_nodes) {
+        return;
+    }
+
+    let Some(cursor) = window.cursor_position() else {
+        return;
+    };
+    let Ok((camera, camera_gt)) = camera_query.single() else {
+        return;
+    };
+
+    // Find the closest star (by screen-pixel distance) within threshold.
+    let mut best: Option<(Entity, f32)> = None;
+    for (entity, transform) in &stars {
+        let Ok(viewport) = camera.world_to_viewport(camera_gt, transform.translation) else {
+            continue;
+        };
+        let dist = viewport.distance(cursor);
+        if dist >= SELECTION_PIXEL_THRESHOLD {
+            continue;
+        }
+        if best.is_none_or(|(_, d)| dist < d) {
+            best = Some((entity, dist));
+        }
+    }
+
+    // Always assign so change detection fires even when clearing the selection.
+    let new_selection = best.map(|(entity, _)| entity);
+    if selected.0 != new_selection {
+        selected.0 = new_selection;
+    }
+}
+
+// ── Visual highlight ────────────────────────────────────────────────────────
+
+/// Smoothly lerp every star's scale toward 1.0 (default) or [`SELECTED_SCALE`]
+/// when it's the currently selected entity.
+pub fn animate_selected_star(
+    selected: Res<SelectedStar>,
+    mut stars: Query<(Entity, &mut Transform), With<StarSystem>>,
+    time: Res<Time>,
+) {
+    // Avoid mutating (and marking-changed) every Transform when nothing is selected
+    // and all stars are already at scale 1.0.
+    let all_default = selected.0.is_none() && stars.iter().all(|(_, t)| t.scale == Vec3::ONE);
+    if all_default {
+        return;
+    }
+
+    let dt = time.delta_secs().min(0.1);
+    let alpha = (dt * SELECTION_LERP_SPEED).clamp(0.0, 1.0);
+
+    for (entity, mut transform) in &mut stars {
+        let target = if selected.0 == Some(entity) {
+            SELECTED_SCALE
+        } else {
+            1.0
+        };
+        let current = transform.scale.x;
+        let next = current + (target - current) * alpha;
+        // Avoid spamming change detection when within float-tolerance of the target.
+        if (next - current).abs() > 1e-4 {
+            transform.scale = Vec3::splat(next);
+        }
+    }
+}
+
+// ── Info panel refresh ──────────────────────────────────────────────────────
+
+/// Rebuild the contents of the right-side info panel whenever the selection
+/// changes. The panel shell stays put; only its children are replaced via
+/// `despawn_related::<Children>()`.
+pub fn refresh_info_panel(
+    mut commands: Commands,
+    selected: Res<SelectedStar>,
+    stars: Query<&StarSystem>,
+    panel_roots: Query<Entity, With<GalaxyInfoPanel>>,
+) {
+    if !selected.is_changed() {
+        return;
+    }
+    let Ok(panel_entity) = panel_roots.single() else {
+        return;
+    };
+
+    // Despawn existing children but keep the panel root itself.
+    commands.entity(panel_entity).despawn_related::<Children>();
+
+    let selected_entity = selected.0;
+    let sys = selected_entity.and_then(|e| stars.get(e).ok());
+
+    commands.entity(panel_entity).with_children(|parent| {
+        parent.spawn((
+            Text::new("Selected System"),
+            TextFont {
+                font_size: 20.0,
+                ..default()
+            },
+            TextColor(Color::srgb(0.82, 0.90, 1.0)),
+            Node {
+                margin: UiRect::bottom(Val::Px(8.0)),
+                ..default()
+            },
+        ));
+
+        match sys {
+            Some(sys) => {
+                let title = sys.name.clone();
+                parent.spawn((
+                    Text::new(title),
+                    TextFont {
+                        font_size: 17.0,
+                        ..default()
+                    },
+                    TextColor(Color::srgb(0.20, 0.83, 0.93)),
+                    Node {
+                        margin: UiRect::bottom(Val::Px(8.0)),
+                        ..default()
+                    },
+                ));
+                spawn_info_line(parent, "Faction", sys.faction);
+                spawn_info_line(parent, "Security", &format!("{:.2}", sys.security));
+                spawn_info_line(parent, "ID", &sys.id);
+            }
+            None => {
+                parent.spawn((
+                    Text::new("Click a star to inspect."),
+                    TextFont {
+                        font_size: 15.0,
+                        ..default()
+                    },
+                    TextColor(Color::srgb(0.55, 0.65, 0.82)),
+                ));
+            }
+        }
+    });
+}
+
+fn spawn_info_line(parent: &mut ChildSpawnerCommands, label: &str, value: &str) {
+    parent
+        .spawn(Node {
+            width: Val::Percent(100.0),
+            flex_direction: FlexDirection::Row,
+            column_gap: Val::Px(8.0),
+            ..default()
+        })
+        .with_children(|row| {
+            row.spawn((
+                Text::new(label),
+                TextFont {
+                    font_size: 14.0,
+                    ..default()
+                },
+                TextColor(Color::srgb(0.55, 0.65, 0.82)),
+                Node {
+                    width: Val::Px(78.0),
+                    ..default()
+                },
+            ));
+            row.spawn((
+                Text::new(value),
+                TextFont {
+                    font_size: 14.0,
+                    ..default()
+                },
+                TextColor(Color::srgb(0.82, 0.90, 1.0)),
+            ));
+        });
+}