Artboards: an artboard's guide label colors its own guides

Each artboard can carry a guide label: one of the layer colors or a
custom RGB. Guides that belong to an artboard draw in its label color,
and guides without an artboard, or on an artboard without a label, keep
the guide color from the preferences. Selected guides still draw in the
selection color.

- Artboard Options has a Guide Label row (None, the layer colors with
  swatches, or Custom with a color swatch).
- artboard.new and artboard.setProps take guideLabel: null, a layer
  color's name, or #rrggbb. Any other value is an error. The inspect
  summary lists guideLabel when one is set.
- The native format saves the label and skips it when unset; a preset
  index outside the layer colors reads as no label.
- artboard.setProps no longer adds an undo step when nothing changes.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/doc/src/node/ids.rs

@@ -56,4 +56,30 @@ impl LayerColor {
             LayerColor::Custom(c) => c,
         }
     }
+
+    /// The color as text: a preset's name from [`LAYER_COLORS`], else `#rrggbb` (lowercase).
+    pub fn label_text(self) -> String {
+        let preset = match self {
+            LayerColor::Preset(i) => LAYER_COLORS.get(usize::from(i)),
+            LayerColor::Custom(_) => None,
+        };
+        preset.map_or_else(
+            || {
+                let [r, g, b] = self.rgb();
+                format!("#{r:02x}{g:02x}{b:02x}")
+            },
+            |(name, _)| (*name).to_string(),
+        )
+    }
+
+    /// The color [`LayerColor::label_text`] names: a preset's name exactly as [`LAYER_COLORS`]
+    /// spells it, or `#rrggbb` with six hex digits. None for anything else.
+    pub fn from_label_text(s: &str) -> Option<Self> {
+        if let Some(i) = LAYER_COLORS.iter().position(|(name, _)| *name == s) {
+            return u8::try_from(i).ok().map(LayerColor::Preset);
+        }
+        let hex = s.strip_prefix('#').filter(|h| h.len() == 6 && h.bytes().all(|b| b.is_ascii_hexdigit()))?;
+        let byte = |k: usize| u8::from_str_radix(&hex[k..k + 2], 16).ok();
+        Some(LayerColor::Custom([byte(0)?, byte(2)?, byte(4)?]))
+    }
 }