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.

README.md

@@ -703,18 +703,24 @@ is what makes branching — the sum of a raw and a filtered channel — expressi
 
 The editor is a compact node graph: each node is a box, its inputs are the squares on its left edge
 and its output the square on its right. Click a box to select it and its parameters appear in the
-form beside the canvas; click an output square and then an input square to wire them; drag a box to
-move it. The graph is a *list* walked in order, so a node can only read the ones above it — a cycle
-is not detected, it is unrepresentable, and an input square is only offered the nodes that qualify.
-The node carrying the channel's value is the **last one** of a graph built top to bottom (as the
-orange dot on the canvas says, and settable with *set as output*): it defaulted to the first node —
-the source — so every operation was computed and discarded, and a gain or a filter produced a
-channel identical to its input. Numeric fields are seeded with their own identity values too, since
+form beside the canvas; **drag from an output square to an input square to wire them** (the old
+two-click protocol is gone); drag a wired input square off its port to detach it; click a wire and
+press `Delete` to remove it; drag a box to move it. **Wheel zooms about the pointer, drag the
+background to pan, `f` to fit, `Ctrl+0` to reset.** `Tidy` in the toolbar lays out all nodes in
+topological columns. The op is chosen when the node is added — the picker appears at the `+` — so
+there is no opless placeholder to build past. The graph is a *list* walked in order, so a node can
+only read the ones above it; a wire drawn right to left re-sorts the list silently, so the graph
+stays evaluable without requiring the user to add nodes in order. A cycle is unrepresentable:
+`math_connect()` checks the trial edge set with Kahn's algorithm before writing anything. The node
+carrying the channel's value is the **last one** of a graph built top to bottom (as the orange dot
+on the canvas says, and settable with *set as output*): it defaulted to the first node — the source
+— so every operation was computed and discarded, and a gain or a filter produced a channel identical
+to its input. Numeric fields are seeded with their own identity values too, since
 `gtk_spin_button_new_with_range()` leaves a spin button at its range's minimum: the offset started
 at -1e6, which turned `x*2` into `x*2 - 2000000`.
 
-Wiring is checked before the channel is created, and the reason is shown rather than the dialog
-closing on the work.
+A live validation line beneath the form says why the graph cannot be saved; Apply is insensitive
+while it is non-empty. The two modal alert boxes that used to appear are gone.
 
 A `Source` node lists only the channels the scope is actually **acquiring**: naming a disabled
 signal would read as no data for ever, and the stream's timestamp signal has no samples at all. The