add briefing icon ID collision validation to qtFRED

FRED2 rejects an icon ID edit that would collide with another icon,
warning and reverting the field, but qtFRED's setIconId() applied the
value unconditionally. Two icons could be given the same ID in a stage
(making find_icon and forward propagation ambiguous), or a global rename
could merge two distinct icon chains by reusing a downstream ID.

setIconId() now mirrors FRED2:

- An ID is a single-icon concept, so it operates on the current icon
  rather than the whole selection (applying one ID to several selected
  icons would itself create a same-stage collision).

- Rejects an ID already used by another icon in the current stage.

- When propagating forward (Change Locally off), rejects an ID already
  used in a later stage, since the rename would merge two icons.

- Otherwise applies the change and renames the icon's forward copies by
  matching the old ID, respecting Change Locally.

It returns false when a change is rejected; the dialog resets the spin
box to the model's value. The spin box is switched to non-keyboard-
tracking so validation runs when the edit is committed (focus-out/Enter)
rather than on every keystroke, avoiding a modal warning on a transient
value mid-typing.

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

Author: Goober5000

qtfred/src/mission/dialogs/BriefingEditorDialogModel.cpp

@@ -785,13 +785,68 @@ int BriefingEditorDialogModel::getIconId() const
 	return s.icons[_currentIcon].id;
 }
 
-void BriefingEditorDialogModel::setIconId(int id)
+bool BriefingEditorDialogModel::setIconId(int id)
 {
-	// briefing icon ids must never be negative
+	auto& briefing = _wipBriefings[_currentTeam];
+	if (briefing.num_stages <= 0 || _currentStage < 0 || _currentStage >= briefing.num_stages)
+		return true;
+
+	auto& stage = briefing.stages[_currentStage];
+	if (_currentIcon < 0 || _currentIcon >= stage.num_icons)
+		return true;
+
+	// an id applies to a single icon, so operate on the current icon rather than the
+	// whole selection; applying one id to several icons would itself create a collision
+	const int oldId = stage.icons[_currentIcon].id;
+	if (id == oldId)
+		return true; // no change
+
+	// briefing icon ids must never be negative (the spin box also enforces this)
 	if (id < 0)
-		return;
+		return false;
+
+	// an icon id must be unique within its stage
+	for (int i = 0; i < stage.num_icons; ++i) {
+		if (i != _currentIcon && stage.icons[i].id == id) {
+			QMessageBox::warning(nullptr,
+				tr("Icon ID"),
+				tr("Icon ID %1 is already used by another icon in this stage.  The ID has not been changed.").arg(id));
+			return false;
+		}
+	}
 
-	applyToSelectedIconsCurrentAndForward([&](brief_icon& ic) { modify(ic.id, id); });
+	// when propagating forward, the new id must not already be used by a different icon in a
+	// later stage, or the global rename would merge two distinct icon chains into one
+	if (!_changeLocally) {
+		for (int st = _currentStage + 1; st < briefing.num_stages; ++st) {
+			const auto& s = briefing.stages[st];
+			for (int i = 0; i < s.num_icons; ++i) {
+				if (s.icons[i].id == id) {
+					QMessageBox::warning(nullptr,
+						tr("Icon ID"),
+						tr("Icon ID %1 is already used in a later stage.  You can only change to that ID "
+						   "locally.  The ID has not been changed.").arg(id));
+					return false;
+				}
+			}
+		}
+	}
+
+	// apply the change to the current icon, and (unless local-only) to the same icon in later stages
+	stage.icons[_currentIcon].id = id;
+	if (!_changeLocally) {
+		for (int st = _currentStage + 1; st < briefing.num_stages; ++st) {
+			auto& s = briefing.stages[st];
+			for (int i = 0; i < s.num_icons; ++i) {
+				if (s.icons[i].id == oldId)
+					s.icons[i].id = id;
+			}
+		}
+	}
+
+	set_modified();
+	modelChanged();
+	return true;
 }
 
 SCP_string BriefingEditorDialogModel::getIconLabel() const

qtfred/src/mission/dialogs/BriefingEditorDialogModel.h

@@ -74,7 +74,8 @@ class BriefingEditorDialogModel : public AbstractDialogModel {
 	vec3d getIconPosition() const;
 	void setIconPosition(const vec3d& pos);
 	int getIconId() const;
-	void setIconId(int id);
+	// returns false if the requested id was rejected (e.g. it collides with another icon)
+	bool setIconId(int id);
 	SCP_string getIconLabel() const;
 	void setIconLabel(const SCP_string& text);
 	SCP_string getIconCloseupLabel() const;

qtfred/src/ui/dialogs/BriefingEditorDialog.cpp

@@ -89,6 +89,10 @@ BriefingEditorDialog::BriefingEditorDialog(FredView* parent, EditorViewport* vie
 	ui->iconCloseupLabelLineEdit->setMaxLength(MAX_LABEL_LEN - 1);
 	ui->voiceFileLineEdit->setMaxLength(MAX_FILENAME_LEN - 1);
 
+	// validate the icon id when the edit is committed (focus-out/Enter) rather than on every
+	// keystroke, so a transient value while typing can't pop a collision warning
+	ui->iconIdSpinBox->setKeyboardTracking(false);
+
 	setupMapWidget();
 	initializeUi();
 	updateUi();
@@ -562,7 +566,12 @@ void BriefingEditorDialog::on_disableGridCheckBox_toggled(bool checked)
 
 void BriefingEditorDialog::on_iconIdSpinBox_valueChanged(int arg1)
 {
-	_model->setIconId(arg1);
+	if (!_model->setIconId(arg1)) {
+		// the id was rejected; reset the spin box to the model's current value
+		ui->iconIdSpinBox->blockSignals(true);
+		ui->iconIdSpinBox->setValue(_model->getIconId());
+		ui->iconIdSpinBox->blockSignals(false);
+	}
 }
 
 void BriefingEditorDialog::on_iconLabelLineEdit_textChanged(const QString& string)