docs: the node editor's gestures, its new probe assertions and its invariants

Each invariant records what went wrong rather than only the rule: a port hit
that lost its node, a hit test at the identity view that asserts nothing, a
`Delete` that must not fall through to the selected node, and a column pitch
too narrow for a wire's own tread. The warning baseline drops to four.

src/help/11-math.txt

@@ -5,15 +5,22 @@ 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; opening one
-shows the graph editor: nodes as boxes, click a box to select it and edit its parameters beside
-the canvas, click an output port then an input port to wire them, drag a box to move it. The
-orange dot marks the output node.
+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.
+
+<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.
 
 <b>Worth knowing</b>
 
 • The graph is evaluated as an ordered list, so a node may only read ones <i>above</i> it. A
-  cycle is impossible by construction rather than rejected by a check.
+  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.
 • 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
@@ -40,10 +47,11 @@ and the last node added becomes the output -- but a graph whose output is left o
 valid and simply passes its input through, which is the usual reason a gain or a filter appears to
 have no effect. <i>Set as output</i> moves the dot.
 
-<b>Save refuses an incomplete graph</b>, and says why: an input that is not connected, no output
-chosen, or a <i>channel</i> node naming a signal the scope is not acquiring. Only the channels being
-acquired are offered, because a source that is not acquired reads as no data for ever -- and a graph
-that reads NaN is harder to diagnose than one that would not save.
+<b>Apply is insensitive while the graph is incomplete.</b> A line beneath the form says why:
+an input that is not connected, no output chosen, or a <i>channel</i> node naming a signal the
+scope is not acquiring. Only the channels being acquired are offered, because a source that is not
+acquired reads as no data for ever — and a graph that reads NaN is harder to diagnose than one that
+would not save.
 
 <b>The filters are real filters.</b> A math channel's samples are produced as the stream arrives,
 once per stored slot, so a low pass carries the history a filter needs. A cutoff typed in hertz is