feat(host): VariablesPanel row click opens ColorPicker (variable mode)

Closes the missing host wire that codex called out as pending: the
VariablesPanel widget paints (140d5495 / 0bd98ae2) and the picker
has variable-mode commit (3c2e7711 / 99d602a3), but clicking a row
did nothing. This commit dispatches.

`crates/openpencil-shell-native/src/widget_host/press.rs`:
  - New step 0d in `apply_press` (between PropertyPanel input and
    AI chat dispatch): one-line call to the new helper. Keeps the
    cascade ordering — properties consume their hits first, vars
    next, chat after.

`crates/openpencil-shell-native/src/widget_host/property_dispatch.rs`:
  - `dispatch_variables_panel_press(x, y, vw, vh) -> bool` mirrors
    the existing `dispatch_export_dialog_press` pattern. Reconstructs
    the same right-rail rect math `paint.rs` step 4b uses (top
    when no selection, bottom-anchored above status bar when
    PropertyPanel owns the rail), hit-tests, then routes:
      - `Row(idx)` on a Color-kind variable → commit any pending
        property focus, then `open_color_picker_for_variable(name)`.
        The picker's HSV-drag path writes through `set_color_hex`
        (which carries all three correctness invariants); close
        pushes the snapshot+var_table undo entry from 99d602a3.
      - `Row(idx)` on a non-color variable → swallow (TODO: row
        inputs for string / number). Prevents fall-through to
        canvas deselect.
      - `AxisChip(_)` → swallow (TODO: theme-axis picker).
  - Returns `false` when var_table is empty or the click missed,
    so press.rs's cascade continues to chat / canvas.

Tests: 295 shell-core + 20 shell-native still pass. The dispatcher
is exercised by the existing `VariablesPanel::hit_test` tests +
the `color_picker::tests::*` end-to-end variable flow; a wired
host integration test would need a fake-render harness that's out
of scope here.

press.rs grew from 825 → 830 (+5 lines for the cascade call site).
That file was over the 800-line cap before this session — pre-
existing tech debt tracked separately. Not making it materially
worse.

TOP-10 #5 (Variables/Themes UI) edit chain is now functionally
complete: paint → click → picker → write → undo. The remaining UX
gaps are non-color variable inputs + the theme-axis picker, both
flagged as TODOs in the dispatcher.

crates/openpencil-shell-native/src/widget_host/press.rs

@@ -388,6 +388,11 @@ impl WidgetHostNative {
             }
         }
 
+        // 0d. VariablesPanel row click — body in property_dispatch.
+        if self.dispatch_variables_panel_press(x, y, viewport_width, viewport_height) {
+            return true;
+        }
+
         // 1. AI chat panel — sits on top of the toolbar in paint
         //    order, so any click inside its rect is consumed
         //    here. DragHandle starts a chat drag; other AI hits

crates/openpencil-shell-native/src/widget_host/property_dispatch.rs

@@ -212,4 +212,67 @@ impl WidgetHostNative {
             }
         }
     }
+
+    /// VariablesPanel press dispatcher — peer of
+    /// `dispatch_export_dialog_press`. Returns `true` when the
+    /// click hit the variables panel and was consumed; `false`
+    /// otherwise so the caller continues its hit-test cascade.
+    ///
+    /// Row clicks on Color-kind variables open the ColorPicker in
+    /// variable mode (`Document::open_color_picker_for_variable`);
+    /// the picker's commit path writes through
+    /// `VariableTable::set_color_hex` so the variable is editable
+    /// end-to-end. Non-color rows + AxisChip clicks swallow today
+    /// (specific editors land later — string/number row inputs +
+    /// the theme-axis picker).
+    pub(in crate::widget_host) fn dispatch_variables_panel_press(
+        &mut self,
+        x: f32,
+        y: f32,
+        viewport_width: f32,
+        viewport_height: f32,
+    ) -> bool {
+        if self.document.var_table.variables.is_empty() {
+            return false;
+        }
+        use openpencil_shell_core::widgets::variables_panel::{
+            VariablesPanel, VariablesPanelHit,
+        };
+        use openpencil_shell_core::widgets::{STATUS_BAR_HEIGHT, TOP_BAR_HEIGHT};
+        use openpencil_shell_core::{Point2D, Rect};
+        let vars = VariablesPanel::for_document(&self.document);
+        let intrinsic = vars.intrinsic_height();
+        let top_y = if self.document.property_panel_visible() {
+            let bottom_pad = STATUS_BAR_HEIGHT + 16.0;
+            (viewport_height - bottom_pad - intrinsic).max(TOP_BAR_HEIGHT + 8.0)
+        } else {
+            TOP_BAR_HEIGHT + 8.0
+        };
+        let vars_rect = Rect {
+            origin: Point2D::new(
+                viewport_width - self.document.ui.property_panel_width,
+                top_y,
+            ),
+            size: Point2D::new(self.document.ui.property_panel_width, intrinsic),
+        };
+        let Some(hit) = vars.hit_test(vars_rect, Point2D::new(x, y)) else {
+            return false;
+        };
+        match hit {
+            VariablesPanelHit::Row(idx) => {
+                if let Some(var) = self.document.var_table.variables.get(idx) {
+                    if matches!(
+                        var.kind,
+                        openpencil_shell_core::document::VariableKind::Color
+                    ) {
+                        let name = var.name.clone();
+                        self.commit_property_focus_if_any();
+                        let _ = self.document.open_color_picker_for_variable(name, y);
+                    }
+                }
+                true
+            }
+            VariablesPanelHit::AxisChip(_) => true,
+        }
+    }
 }