layer states: restore into the active layout viewport (#1796)

Restoring a layer state while a layout viewport is active now applies it
to that viewport alone, as viewport freezes and viewport overrides for
colour, linetype, lineweight and transparency (where they differ from the
layer, or an override is already there). On/off, lock and plot have no
per-viewport form and still apply to the drawing.

Viewport colour overrides are packed as the format does (0xC3 index,
0xC2 true colour); reading them now honours that type byte, so an index
override from a file no longer shows as a near-black true colour.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/view/render.rs

@@ -3433,6 +3433,32 @@ pub(in crate::scene) fn render_style_for(
     render_style_for_viewport(document, e, None)
 }
 
+/// A viewport layer colour override as the format packs it (code 420): the
+/// top byte says what follows — 0xC3 an index colour in the low byte, 0xC2 a
+/// 24-bit true colour, 0xC0 / 0xC1 ByLayer / ByBlock. A bare 24-bit value is
+/// read as true colour.
+pub(crate) fn layer_override_color(value: i32) -> AcadColor {
+    match (value as u32) >> 24 {
+        0xC0 => AcadColor::ByLayer,
+        0xC1 => AcadColor::ByBlock,
+        0xC3 => AcadColor::Index((value & 0xFF) as u8),
+        _ => AcadColor::from_true_color_value(value),
+    }
+}
+
+/// [`layer_override_color`] the other way round; `None` for a colour an
+/// override cannot hold.
+pub(crate) fn pack_layer_override_color(color: &AcadColor) -> Option<i32> {
+    let packed = match color {
+        AcadColor::Index(index) => 0xC300_0000u32 | *index as u32,
+        AcadColor::Rgb { r, g, b } => {
+            0xC200_0000u32 | (*r as u32) << 16 | (*g as u32) << 8 | *b as u32
+        }
+        _ => return None,
+    };
+    Some(packed as i32)
+}
+
 fn viewport_override(
     document: &CadDocument,
     layer_name: &str,
@@ -3473,7 +3499,7 @@ pub(crate) fn render_style_for_common_viewport(
                         codec::objects::KnownXRecordKind::LayerViewportColorOverride,
                     )
                     .and_then(|value| value.as_i32())
-                    .map(AcadColor::from_true_color_value)
+                    .map(layer_override_color)
                 })
                 .flatten();
         let resolved = if *ec == AcadColor::ByLayer {
@@ -3622,7 +3648,7 @@ pub(crate) fn layer_render_style_viewport(
             codec::objects::KnownXRecordKind::LayerViewportColorOverride,
         )
         .and_then(|value| value.as_i32())
-        .map(AcadColor::from_true_color_value)
+        .map(layer_override_color)
     } else {
         None
     };