refactor: split four oversized files into submodule trees

Break up the four giant files (~29k lines total) into focused submodule
directories, one concern per file. Pure mechanical moves — behavior is
unchanged.

- app/update.rs (9650) -> app/update/{mod,viewport,command,style,file,
  dynamic,dialog,util}. The 384-arm update_inner match stays in mod.rs
  with thin delegating arms; the 66 fattest arm bodies move to topic
  methods. Arm set preserved exactly (384 -> 384).
- scene/mod.rs (9017) -> scene/{mod,entity,tess,hittest,layout,paper,
  mspace,project,selection,modify,group_layer,preview}. The impl Scene
  body is split across files via inherent-impl-per-file; struct, ctor,
  fields and load-time helpers stay in the root. fn count 123 -> 123.
- app/commands.rs (6082) -> app/commands/* grouped by command family.
  The 233-arm dispatch match becomes source-ordered per-family
  dispatch_* handlers, preserving first-match precedence (233 -> 233).
- app/view.rs (4686) -> app/view/{mod,overlay,modal,viewcube,controls}.

Largest file drops from 9650 to 3343. Builds clean; app launches and
loads plugins with no panic.

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

src/app/view/viewcube.rs

@@ -0,0 +1,195 @@
+use super::super::Message;
+use crate::scene::{VIEWCUBE_PX, VIEWCUBE_REGION_PX};
+use iced::widget::{
+    button, column, container, mouse_area, pick_list, row, stack,
+    Space,
+};
+use iced::{Background, Border, Color, Element, Fill, Theme};
+
+// ── Render-mode picker ──────────────────────────────────────────────────────
+
+/// Top-left viewport control bar: a single dark chip holding (optionally) the
+/// horizontal/vertical split buttons, the render-mode picker, and the grid /
+/// grid-snap toggles. `include_split` is off for paper-space viewports, which
+/// have no model-tile splitting. Grid / snap reflect the active viewport's
+/// state and emit `ToggleGrid` / `ToggleGridSnap`.
+// ── ViewCube navigation controls (home / roll / nudge / UCS) ───────────────
+
+/// Place `el` at pixel offset (x, y) inside a Fill layer (top-left origin).
+fn vc_place<'a>(x: f32, y: f32, el: Element<'a, Message>) -> Element<'a, Message> {
+    column![
+        Space::new().height(iced::Length::Fixed(y.max(0.0))),
+        row![Space::new().width(iced::Length::Fixed(x.max(0.0))), el],
+    ]
+    .width(Fill)
+    .height(Fill)
+    .into()
+}
+
+/// Borderless square icon button used by the ViewCube nav controls.
+fn vc_btn<'a>(content: Element<'a, Message>, size: f32, msg: Message) -> Element<'a, Message> {
+    button(
+        container(content)
+            .width(iced::Length::Fixed(size))
+            .height(iced::Length::Fixed(size))
+            .center_x(iced::Length::Fixed(size))
+            .center_y(iced::Length::Fixed(size)),
+    )
+    .padding(0)
+    .on_press(msg)
+    .style(|_: &Theme, status| iced::widget::button::Style {
+        background: Some(Background::Color(match status {
+            iced::widget::button::Status::Hovered | iced::widget::button::Status::Pressed => Color {
+                r: 0.45,
+                g: 0.62,
+                b: 0.95,
+                a: 0.30,
+            },
+            _ => Color::TRANSPARENT,
+        })),
+        border: Border {
+            radius: 3.0.into(),
+            ..Default::default()
+        },
+        ..Default::default()
+    })
+    .into()
+}
+
+/// Overlay of home / roll / nudge controls sized to the whole nav region, so
+/// the caller can position it exactly like the cube hit area.
+pub(super) fn viewcube_nav_controls<'a>() -> Element<'a, Message> {
+    use crate::scene::NudgeDir;
+    use crate::ui::icons;
+    let tint = Color {
+        r: 0.86,
+        g: 0.89,
+        b: 0.96,
+        a: 1.0,
+    };
+    let r = VIEWCUBE_REGION_PX;
+    let c = r * 0.5;
+    let cube_half = VIEWCUBE_PX as f32 * 0.36; // VIEWCUBE_PX * VIEWCUBE_SCALE
+    let nr = cube_half + 8.0; // nudge triangle distance from centre
+    const BTN: f32 = 18.0;
+    const TRI: f32 = 13.0;
+    let ctr = |cx: f32, cy: f32, s: f32| (cx - s * 0.5, cy - s * 0.5);
+
+    // Home top-left, roll arrows top-right.
+    let (rax, ray) = (r - 2.0 * BTN - 4.0, 2.0);
+    let (rbx, rby) = (r - BTN - 2.0, 2.0);
+    // Nudge triangles pointing inward at the four cube faces.
+    let (tux, tuy) = ctr(c, c - nr, TRI);
+    let (tdx, tdy) = ctr(c, c + nr, TRI);
+    let (tlx, tly) = ctr(c - nr, c, TRI);
+    let (trx, try_) = ctr(c + nr, c, TRI);
+
+    // Bottom layer: the cube/cardinal hit area covering the whole region. The
+    // control buttons sit ABOVE it in the same stack, so a click on a button is
+    // caught by the button while a click on the cube (or empty space) falls
+    // through to this mouse_area → ViewportClick. Moves keep cursor_pos current.
+    let cube_hit = mouse_area(
+        Space::new()
+            .width(iced::Length::Fixed(r))
+            .height(iced::Length::Fixed(r)),
+    )
+    .on_move(Message::CursorMoved)
+    .on_press(Message::ViewportClick);
+
+    let controls = stack![
+        cube_hit,
+        vc_place(
+            3.0,
+            3.0,
+            vc_btn(icons::home(15.0, tint), BTN, Message::ViewCubeHome)
+        ),
+        vc_place(
+            rax,
+            ray,
+            vc_btn(icons::undo(14.0, tint), BTN, Message::ViewCubeRoll(false))
+        ),
+        vc_place(
+            rbx,
+            rby,
+            vc_btn(icons::redo(14.0, tint), BTN, Message::ViewCubeRoll(true))
+        ),
+        vc_place(
+            tux,
+            tuy,
+            vc_btn(
+                icons::arrow_down(11.0, tint),
+                TRI,
+                Message::ViewCubeNudge(NudgeDir::Up)
+            )
+        ),
+        vc_place(
+            tdx,
+            tdy,
+            vc_btn(
+                icons::arrow_up(11.0, tint),
+                TRI,
+                Message::ViewCubeNudge(NudgeDir::Down)
+            )
+        ),
+        vc_place(
+            tlx,
+            tly,
+            vc_btn(
+                icons::arrow_right(11.0, tint),
+                TRI,
+                Message::ViewCubeNudge(NudgeDir::Left)
+            )
+        ),
+        vc_place(
+            trx,
+            try_,
+            vc_btn(
+                icons::arrow_left(11.0, tint),
+                TRI,
+                Message::ViewCubeNudge(NudgeDir::Right)
+            )
+        ),
+    ];
+
+    container(controls)
+        .width(iced::Length::Fixed(r))
+        .height(iced::Length::Fixed(r))
+        .into()
+}
+
+/// The WCS / named-UCS selector shown under the cube.
+pub(super) fn viewcube_ucs_picker<'a>(current: String, names: Vec<String>) -> Element<'a, Message> {
+    let light = Color {
+        r: 0.85,
+        g: 0.87,
+        b: 0.93,
+        a: 1.0,
+    };
+    let mut options = vec!["WCS".to_string()];
+    options.extend(names);
+    let selected = if current.is_empty() {
+        "WCS".to_string()
+    } else {
+        current
+    };
+    pick_list(options, Some(selected), Message::SetViewcubeUcs)
+        .text_size(11)
+        .padding([2, 6])
+        .style(move |_: &Theme, _| iced::widget::pick_list::Style {
+            background: Background::Color(Color {
+                r: 0.16,
+                g: 0.17,
+                b: 0.20,
+                a: 0.92,
+            }),
+            border: Border {
+                radius: 3.0.into(),
+                ..Default::default()
+            },
+            text_color: light,
+            placeholder_color: light,
+            handle_color: light,
+        })
+        .into()
+}
+