feat: switching a signal off takes its traces off every plot

The switch in the signal list means "do not acquire this", and until now a trace of it
stayed on its plot drawing nothing: a slot that holds one of the four a plot has, reads
`--`, and blocks the drop that would fill it with something live.

`on_signal_switch()` now clears every slot that holds that signal, by the **signal name**
the slot remembers -- never the channel index, which is what the enable change renumbers --
and says what it did: `'sine' is off: 1 trace left the plots`. The follow-ups (a relayout,
the panels' chips, the layout save) happen once rather than per slot.

Re-enabling deliberately does not put the traces back: the arrangement is the user's, and
guessing where they were is worse than leaving them off. Say the word if you want that
direction too.

[enable-all-probe] reads it end to end -- it enables every signal, assigns the channels
across the rows, then switches one off *through its own acquire tick*:

    [enable-all-probe] switched 'sine' off: traces_left=0 (want 0) notice=''sine' is off: 1 trace left the plots'

which is also why the notice shares the status line's warning styling from the previous
commit: the trace leaving the plots is something the user should see happen.

docs/claude/invariants-plot-and-layout.md

@@ -173,3 +173,12 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   because `relayout_areas()` is the one place that builds a figure and re-points the area at it.
   `UDPSCOPE_PROBE_COLOR_MS` reads the result back out of `plot::Plot::trace()` and out of the
   chip's own markup, since asking the resolver twice would assert nothing.
+
+- **Switching a signal off takes its traces off every plot.** The switch means "do not acquire this", and a
+  trace left behind draws nothing: it holds one of the four slots a plot has, reads `--`, and blocks the drop
+  that would fill it with something live. `on_signal_switch()` clears every slot by the **signal name** the
+  slot remembers -- never the channel index, which is exactly what the enable change renumbers -- and says what
+  it did in a notice, because a trace that vanishes silently is its own kind of confusing. The follow-ups (a
+  relayout, the panels' chips, the layout save) are done once by the caller rather than per slot. Re-enabling
+  does **not** put the traces back: the arrangement is the user's, and guessing where they were is worse than
+  leaving them off.