StimulusProducer: add duplicate-quale guard and attach-identity buffer lookup.

Provide ensureNoDuplicateQualeIface and getAttachedStimulusBufferByAttachIdentity
so session-scoped stimBuff plugins can reject duplicate quales and detach by
stable DAP line identity rather than full spec equality.

Co-authored-by: Cursor <cursoragent@cursor.com>

include/user/stimulusProducer.h

@@ -56,6 +56,10 @@ public:
 	virtual std::shared_ptr<StimulusBuffer> getAttachedStimulusBuffer(
 		const std::shared_ptr<device::DeviceAttachmentSpec>& spec) const;
 
+	std::shared_ptr<StimulusBuffer> getAttachedStimulusBufferByAttachIdentity(
+		const std::string& deviceIdentifier,
+		const std::string& qualeIfaceApi) const;
+
 	virtual std::shared_ptr<StimulusBuffer> getOrCreateAttachedStimulusBuffer(
 		const std::shared_ptr<device::DeviceAttachmentSpec>
 			&deviceAttachmentSpec) = 0;
@@ -72,6 +76,10 @@ public:
 	// Check if any attached buffer has the specified qualeIfaceApi
 	bool hasBufferWithQualeIfaceApi(const std::string& qualeIfaceApi) const;
 
+	/** Reject a second buffer for the same qualeIface on this producer session. */
+	void ensureNoDuplicateQualeIface(
+		const std::string& qualeIfaceApi) const;
+
 protected:
 	// Virtual functions for derived classes to override
 	virtual int getStopDelayMs() const