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>

docs/claude/invariants-session-and-export.md

@@ -6,7 +6,10 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
 
 - **The session is the layout.** `LayoutState` persists what makes a reopened scope look like the
   one that was closed: the window, the rows and their slot signals, the calibration, **the enabled
-  signals**, **the math channels with their graphs** and **the measurements pane**. Signal identity
+  signals**, **the chosen trace colours** (`[[color]]`, `name` + `rgb = "#RRGGBB"` — only the
+  *overrides*, so a stream's new signals still take the positional palette and a saved session does
+  not freeze a colour that is meant to follow the signal list), **the math channels with their
+  graphs** and **the measurements pane**. Signal identity
   is the *name* throughout (a CONFIG renumbers channels), 'enabled' is read off the channels the
   model built -- an enabled signal is exactly one that produced a channel -- and `apply_layout()`
   installs the graphs **before** assigning the rows, since a row may hold a math channel's name and