feat: a signal's chip is clickable, so a trace's colour is the user's to choose

Clicking the coloured chip in the signal list opens a GtkColorDialog and the
colour it returns overrides that signal's palette entry. Held by name, like
every other identity here, and persisted as a session `[[color]]` table.

RowSet::signal_color() becomes the one resolver: install() builds the trace
through it and all three chip sites read it, because a chip exists to say which
trace a row is about and an override is exactly the state in which two routes to
a colour would disagree. A grey chip names no trace and so is not clickable, and
the way back is `Default colour` in the signal's menu -- a setting with no undo
is a trap.

The gesture sits on the existing label rather than in a GtkButton: padding on
this row is what made the signal list twice as tall once. It claims its sequence
so a click on the chip cannot become the start of a row drag.

UDPSCOPE_PROBE_COLOR_MS reads the result out of plot::Plot::trace() and out of
the chip's own markup, with an override in no palette slot, so neither leg can
pass by asking the resolver what it already said.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/layout_store.cpp

@@ -239,13 +239,14 @@ bool load_layout(const std::string& path, LayoutState& out) {
   st.window_s = 0.0;
   st.rows.clear();
   st.calibration.clear();
+  st.colors.clear();
   st.enabled.clear();
   st.math.clear();
   st.measures.clear();
 
   std::string line;
   // What the last `[[...]]` header opened, so keys land in the right record.
-  enum class Section { Top, Plot, Calibration, Math, Measure } section = Section::Top;
+  enum class Section { Top, Plot, Calibration, Color, Math, Measure } section = Section::Top;
   int plot_count = 0;
   bool plot_section_seen = false;
   while (std::getline(in, line)) {
@@ -259,6 +260,9 @@ bool load_layout(const std::string& path, LayoutState& out) {
       } else if (t == "[[calibration]]") {
         section = Section::Calibration;
         st.calibration.emplace_back();
+      } else if (t == "[[color]]") {
+        section = Section::Color;
+        st.colors.emplace_back();
       } else if (t == "[[math]]") {
         section = Section::Math;
         st.math.emplace_back();
@@ -378,6 +382,23 @@ bool load_layout(const std::string& path, LayoutState& out) {
         } catch (...) {
         }
       }
+    } else if (section == Section::Color && !st.colors.empty()) {
+      if (key == "name") {
+        st.colors.back().name = strip_quote(value);
+      } else if (key == "rgb") {
+        // `"#RRGGBB"`, so the file says what it means to a reader hand-editing it -- a bare decimal
+        // would be the one number in this file nobody could check by eye. Anything unparseable
+        // leaves the entry at 0, and `apply_layout()` drops an entry with no name anyway.
+        const std::string hex = strip_quote(value);
+        const std::size_t at = hex.find('#');
+        try {
+          st.colors.back().rgb =
+              static_cast<std::uint32_t>(std::stoul(
+                  at == std::string::npos ? hex : hex.substr(at + 1), nullptr, 16)) &
+              0x00FFFFFFu;
+        } catch (...) {
+        }
+      }
     }
   }
   if (!plot_section_seen && plot_count > 0) {
@@ -422,6 +443,11 @@ bool save_layout(const std::string& path, const LayoutState& state) {
     o << "\n[[calibration]]\nname = " << toml_escape(cal.name) << "\ngain = " << cal.gain
       << "\noffset = " << cal.offset << "\n";
   }
+  for (const auto& color : state.colors) {
+    char rgb[16];
+    std::snprintf(rgb, sizeof(rgb), "\"#%06X\"", color.rgb & 0x00FFFFFFu);
+    o << "\n[[color]]\nname = " << toml_escape(color.name) << "\nrgb = " << rgb << "\n";
+  }
   for (const MathChannel& m : state.math) {
     o << "\n[[math]]\nname = " << toml_escape(m.name) << "\nunit = " << toml_escape(m.unit)
       << "\nenabled = " << (m.enabled ? "true" : "false") << "\noutput = " << m.output