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/probe-readings.md

@@ -1243,3 +1243,15 @@ second is that it *said so* -- the message and the stock `.warning` class the st
 notice is fresh; the third is the composed line the user reads, which is why the label is only checked here
 and not in the second: the status line folds the notice into itself on its own cadence, so a beat after the
 refusal it may still hold the previous text.
+
+## `[enable-all-probe]` and switching a signal off
+
+```
+[enable-all-probe] 4 channel(s) enabled and assigned to plots
+[enable-all-probe] switched 'sine' off: traces_left=0 (want 0) notice=''sine' is off: 1 trace left the plots'
+```
+
+`UDPSCOPE_ENABLE_ALL_MS` enables every signal and assigns the acquired channels across the rows, then
+switches one off **through its own acquire tick in the signal list** -- the tick is a box inside the row, which
+is the shape `signal_tick_visible()` walks -- and reads the plots back. `traces_left=0` is the rule: a disabled
+signal leaves every plot rather than staying as a slot that draws nothing, and the notice is what says so.