fix(geometry): canvas_region shrinks for VariablesPanel too (codex BLOCK)

Codex stop-gate flagged: with no selection, the right-rail
`VariablesPanel` was painted (commit 0bd98ae2 wired the paint
step) but `canvas_region` still returned the full viewport width
because its `has_property` gate only checked `property_panel_
visible()` — which is false without a selection. The canvas
paint pass (Step 5) then extended over the right rail and
overpainted the Variables panel, defeating the visibility fix
the previous commit was supposed to deliver.

`crates/openpencil-shell-core/src/document/mutators.rs`:
  - New `Document::right_rail_visible() -> bool` is the unified
    gate: true when either `property_panel_visible()` is true or
    `var_table.variables` is non-empty. Future right-rail widgets
    (Components, Themes header, etc.) extend this method instead
    of patching every caller.

`crates/openpencil-shell-native/src/widget_host/geometry.rs`:
  - `canvas_region` swapped from `property_panel_visible()` to
    `right_rail_visible()`. The shrunk-canvas branch no longer
    requires a selection — any rail-occupying widget reserves the
    column.

Test (1 added, 275 shell-core total):
  - `right_rail_visible_tracks_property_panel_and_variables`
    covers the four state combinations:
      empty doc                       → rail hidden
      var-table-only (no selection)   → rail shown (codex repro)
      selection-only (no vars)        → rail shown (legacy gate)
      cleared selection + cleared vars→ rail hidden again

20 shell-native + 54 desktop + 275 shell-core tests pass.

crates/openpencil-shell-core/src/document/mutators.rs

@@ -387,6 +387,18 @@ impl Document {
         }
     }
 
+    /// True when ANY widget occupies the right rail today —
+    /// PropertyPanel (gated on selection) or VariablesPanel
+    /// (gated on a non-empty var table). `canvas_region` uses
+    /// this to size the canvas so it doesn't paint over the
+    /// rail content (codex BLOCK: `no-selection VariablesPanel
+    /// is painted under the canvas` — without this gate the
+    /// canvas extended full-width when nothing was selected,
+    /// hiding the Variables panel).
+    pub fn right_rail_visible(&self) -> bool {
+        self.property_panel_visible() || !self.var_table.variables.is_empty()
+    }
+
     /// Union of `aggregate_bounds` across selected nodes on the
     /// active page. Backs the multi-select panel's X/Y/W/H.
     pub fn selection_bounds(&self) -> Option<crate::Rect> {

crates/openpencil-shell-core/src/document/tests_geometry.rs

@@ -238,6 +238,34 @@ fn property_panel_visible_for_single_and_multi_selection() {
     assert!(doc.property_panel_visible());
 }
 
+#[test]
+fn right_rail_visible_tracks_property_panel_and_variables() {
+    use crate::document::{Variable, VariableKind, VariableScalar, VariableValue};
+    // No selection + empty var table → no right rail.
+    let mut doc = Document::empty();
+    assert!(!doc.right_rail_visible());
+
+    // Variables-only document (typical for `.op` files with themes
+    // loaded but nothing selected on first paint) — rail occupied,
+    // canvas_region must shrink so the panel isn't painted over.
+    // Codex BLOCK: canvas previously extended full-width here.
+    doc.var_table.variables.push(Variable {
+        name: "color-1".into(),
+        kind: VariableKind::Color,
+        value: VariableValue::Scalar(VariableScalar::Str("#ff0000".into())),
+    });
+    assert!(doc.right_rail_visible());
+
+    // Selection-only document → rail occupied (legacy gate path).
+    let mut doc2 = Document::sample();
+    assert!(doc2.right_rail_visible());
+
+    // Clearing both selection + vars hides the rail again.
+    doc2.clear_selection();
+    doc2.var_table.variables.clear();
+    assert!(!doc2.right_rail_visible());
+}
+
 #[test]
 fn property_panel_visible_hides_stale_single_anchor() {
     // Codex stop-hook BLOCK guard: the panel paint gate

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

@@ -257,8 +257,12 @@ impl WidgetHostNative {
         } else {
             0.0
         };
-        let has_property = self.document.property_panel_visible();
-        let canvas_right = if has_property {
+        // Any right-rail widget (PropertyPanel OR VariablesPanel)
+        // claims the rail width. Using `right_rail_visible` keeps
+        // canvas + rail mutually exclusive so the canvas never paints
+        // over an open Variables panel (codex BLOCK fix).
+        let rail_occupied = self.document.right_rail_visible();
+        let canvas_right = if rail_occupied {
             viewport_w - self.document.ui.property_panel_width
         } else {
             viewport_w