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/verification.md

@@ -161,6 +161,11 @@ precedent, not an exception.
   stops while paused and "how long have I been looking at this" is a question about the user, not
   about the stream; it is written through `format_elapsed()` so the field steps to a coarser unit
   instead of growing digits (see the layout rule).
+- `UDPSCOPE_PROBE_COLOR_MS` — recolour one plotted signal and read the colour back out of the
+  trace and the chip, round-trip it through the session writer, then clear it. Schedule
+  `UDPSCOPE_ENABLE_ALL_MS` before it: with nothing bound it has no trace to recolour and asserts
+  nothing. Its four lines are in
+  [probe-readings.md](probe-readings.md).
 - `UDPSCOPE_EXPORT_MS` (+ optional `UDPSCOPE_EXPORT_PATH`, default
   `/tmp/udpscope-export`) — collect the export table and write it both ways, skipping
   only the file chooser.