Reject non-finite element/terminal coordinates on load (fixes #781, #782)

QString::toDouble() reports a successful conversion for "nan"/"inf"/
"-inf" -- confirmed directly -- so the existing conv_ok checks in
Element::valideXml() and Terminal::valideXml() never caught a
non-finite x/y. A NaN-positioned element reaching the scene can hang
QGraphicsScene::addItem() forever: an existing conductor's itemChange()
runs a collision test (calculateTextItemPosition() ->
QGraphicsItem::collidesWithPath()) whose underlying QPathClipper spins
without terminating when fed a NaN-valued QPainterPath, since NaN
breaks the ordering comparisons the clipping algorithm's termination
depends on (#781). Short of that, a non-finite value that doesn't
happen to trigger a collision test simply gets written straight back
out on save with nothing to stop it (#782).

Element::valideXml() and Terminal::valideXml() now also check
qIsFinite() on the parsed x/y, rejecting the whole item the same way a
missing attribute already does. DynamicElementTextItem::fromXml() has
no such reject-the-item gate (void return, no caller check), so its x/y
are clamped to 0 instead -- the same fallback the attribute lookup
already uses when x/y is missing entirely.

Verified against both original findings' exact repro steps:
- #781: Habitat-Unifilaire.qet with x="nan" on one element -- hung
  (SIGTERM'd by a 25s timeout) on an unfixed build, resaves cleanly
  (exit 0) on this one.
- #782: grafcet.qet with y="nan" on a dynamic_elmt_text -- the value
  passed straight through to the resaved file on an unfixed build;
  clamped to 0 on this one. The specific field is now stable (y="0" on
  two consecutive resaves) where it read "nan" both times before.
  (grafcet.qet has an unrelated, already-known, unmerged fix
  (PR #779) for element/terminal-order non-determinism, so a whole-file
  diff across resaves still differs for reasons unconnected to this
  change -- checked the specific once-NaN field in isolation instead.)
- Also checked -inf on affuteuse_250h.qet: same rejection, same result.

Full qet-dbcheck.py sweep of the unmutated example corpus (23
projects), 0 regressions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sources/qetgraphicsitem/dynamicelementtextitem.cpp

@@ -30,6 +30,7 @@
 #include <QTimer>
 #include <QDomDocument>
 #include <QDomElement>
+#include <QtCore/qnumeric.h>
 #include <QGraphicsSceneMouseEvent>
 
 /**
@@ -221,8 +222,16 @@ void DynamicElementTextItem::fromXml(const QDomElement &dom_elmt)
 		//Force the update of the displayed text
 	setTextFrom(m_text_from);
 	
-	QGraphicsTextItem::setPos(dom_elmt.attribute("x", QString::number(0)).toDouble(),
-							  dom_elmt.attribute("y", QString::number(0)).toDouble());
+		//QString::toDouble() accepts "nan"/"inf"/"-inf" as a successful
+		//conversion, so a corrupted or hand-edited file can hand this a
+		//non-finite position -- fall back to 0 the same way a missing
+		//attribute already does, rather than letting it reach the scene
+		//(see the matching comment in Element::valideXml(), the fromXml()
+		//gate a plain element's position goes through; this text item has
+		//no such gate to reject the whole item at, so it clamps instead).
+	double x = dom_elmt.attribute("x", QString::number(0)).toDouble();
+	double y = dom_elmt.attribute("y", QString::number(0)).toDouble();
+	QGraphicsTextItem::setPos(qIsFinite(x) ? x : 0, qIsFinite(y) ? y : 0);
 }
 
 /**