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.

docs/claude/verification.md

@@ -13,12 +13,14 @@ xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=8000 UDPSCOPE_ENABLE_ALL_MS=1000 \
   ./build/udpscope --server 127.0.0.1 --port 8000
 ```
 
-Two baselines every run is read against. A clean build emits **five** warnings and no more: two
-`-Wmissing-field-initializers` on `MathNode::source`, one unused `output_built`, and
-`-Woverlength-strings` on **both** generated GResource `.c` files — the help pages' and the
-icons', whose blob crossed C99's 4095-byte string-literal limit when the two app icons joined the
-four action ones (a note about a generated file, which GCC itself supports). And an exit-dump run
-emits **no** `Gtk-CRITICAL` at all. A sixth warning, or any critical, is a finding.
+Two baselines every run is read against. A clean build emits **four** warnings and no more: two
+`-Wmissing-field-initializers` on `MathNode::source` and `-Woverlength-strings` on **both**
+generated GResource `.c` files — the help pages' and the icons', whose blob crossed C99's
+4095-byte string-literal limit when the two app icons joined the four action ones (a note about a
+generated file, which GCC itself supports). The fifth warning that used to appear here was an
+unused `output_built` in `on_math_apply()` — Task 6's live validation check reads that value, so
+the variable is no longer unused. And an exit-dump run emits **no** `Gtk-CRITICAL` at all. A
+fifth warning, or any critical, is a finding.
 
 That second baseline used to be "exactly two, `gtk_widget_get_height` and `gtk_widget_get_width`,
 from the dump asking a destroyed widget for its size", and tolerating them was a mistake: the dump