fix: a wire is picked by distance and a refusal is spoken, so the editor answers

A review of the whole node editor. The hit test measured the distance to the
24 curve samples rather than to the segments between them, so a wire spanning
several rows was unpickable between two samples -- the probe now places its
boxes 280 units apart and prints the nearest sample's own 8.4 against a 3.0
tolerance, which the point test would have missed. A refused connection said
nothing: the message is now carried in `MathDialog::refusal` and shown once by
the single writer of the label, and a self-wire is refused through the real
drag-end path instead of only through `math_connect()`.

Two defects were found only by the new assertions. `math_dialog_sync()`
re-entered itself -- `gtk_drop_down_set_selected()` emits `notify::selected`
for a programmatic set too, and the handler answers by syncing again, which
consumed the refusal before it could be drawn -- so the body is now guarded
one level deep. And a deleted node left `selected_wire` naming an index that
afterwards belongs to a different wire, so `Delete` cleared an input on the
wrong node.

A node is placed in the first free grid cell rather than at the end of the
list's own cell, so a node added after a delete no longer lands on top of a
survivor; the label ellipsizes with a tooltip rather than wrapping, because a
wrapped line makes the dialog's height depend on the text in it.

src/help/11-math.txt

@@ -5,22 +5,24 @@ nodes, operations, one output — and udpscope then treats the result as an ordi
 
 <b>How to reach it</b>
 
-The math button in the toolbar, or <tt>g</tt>. The pane's <tt>+</tt> adds a channel — a picker
-chooses the op — and opening one shows the graph editor: nodes as boxes, click a box to select it
-and edit its parameters beside the canvas.
+The math button in the toolbar, or <tt>g</tt>. The pane's <tt>+</tt> adds a channel, and opening
+one shows the graph editor: nodes as boxes, click a box to select it and edit its parameters beside
+the canvas. Inside the editor, <i>add node</i> is a menu button whose popover picks the op, so the
+node arrives as the operation you asked for.
 
 <b>Graph gestures:</b> <b>drag</b> from an output port to an input port to wire them; drag a
 wired input port off its connection to detach it; click a wire and press <tt>Delete</tt> to remove
 it; drag a box to move it. <b>Wheel</b> zooms about the pointer. <b>Drag the background</b> to
-pan. <tt>f</tt> fits all nodes into view; <tt>Ctrl+0</tt> resets the view. <tt>Tidy</tt> in the
-toolbar lays the graph out in topological columns. The orange dot marks the output node; click
-<i>set as output</i> to move it.
+pan. <tt>f</tt> fits all nodes into view; <tt>Ctrl+0</tt> resets the view. <i>tidy</i>, the button
+beside <i>fit</i> in the editor, lays the graph out in topological columns and frames it. The orange
+dot marks the output node; click <i>set as output</i> to move it.
 
 <b>Worth knowing</b>
 
 • The graph is evaluated as an ordered list, so a node may only read ones <i>above</i> it. A
   wire drawn right to left re-sorts the list silently so the graph stays valid. A cycle is
-  unrepresentable: the connection is refused rather than created.
+  unrepresentable: the connection is refused rather than created, and the line beneath the form
+  says why — a loop, or a node asked to read itself.
 • A derived channel <i>is</i> a channel. It appears in the signal panel, can be plotted,
   measured, exported, and used as a source for another math channel.
 • The math runs on the <b>stored</b> samples — decimated and averaged — so a cutoff you type in