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.

docs/claude/verification.md

@@ -17,10 +17,14 @@ Two baselines every run is read against. A clean build emits **four** warnings a
 `-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.
+generated file, which GCC itself supports). The fifth warning that used to appear here was an unused
+`output_built` in `math_probe()`; it was **deleted** as dead code — redundant with the
+`output_after_add` already printed on the same line — and that deletion is what dropped the baseline
+from five to four. Nothing started reading it. And an exit-dump run emits **no** `Gtk-CRITICAL` at
+all. A fifth warning, or any critical, is a finding.
+
+An *incremental* build reads **two** of those four: the generated GResource `.c` files are not
+recompiled when only `src/` changed, so their `-Woverlength-strings` do not reappear.
 
 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