Add DYNAMIC_DATA placeholder for raw streamed template content

Support registering dynamic String/progmem data without re-parsing as
nested templates, used by WiFiManager for composed portal stylesheets.

include/DeviceFrameworkPlaceholderRegistry.h

@@ -90,6 +90,7 @@ public:
      * @return true if registered successfully
      */
     bool registerRamData(const char* name, PlaceholderDataGetter getter);
+    bool registerDynamicData(const char* name, const DynamicDataDescriptor* descriptor);
     bool registerDynamicTemplate(const char* name, const DynamicTemplateDescriptor* descriptor);
     bool registerConditional(const char* name, const ConditionalDescriptor* descriptor);
     bool registerIterator(const char* name, const IteratorDescriptor* descriptor);
@@ -124,6 +125,7 @@ public:
     // Static helper functions for length calculation
     static size_t getProgmemLength(const void* data);
     static size_t getRamLength(const void* data);
+    static size_t getDynamicDataLength(const DynamicDataDescriptor* descriptor, const char* data);
     static size_t getDynamicTemplateLength(const DynamicTemplateDescriptor* descriptor, const char* templateData);
     
 private:
@@ -138,6 +140,8 @@ private:
                                  uint8_t* dest, size_t maxLen);
     static size_t copyRamData(PlaceholderDataGetter getter, size_t offset, 
                              uint8_t* dest, size_t maxLen);
+    static size_t copyDynamicData(const DynamicDataDescriptor* descriptor, size_t offset,
+                                 uint8_t* dest, size_t maxLen);
 };
 
 #endif // DEVICEFRAMEWORK_PLACEHOLDER_REGISTRY_H

include/DeviceFrameworkTemplateTypes.h

@@ -31,6 +31,7 @@ enum class PlaceholderType {
     PROGMEM_DATA,      // Large PROGMEM data (CSS, JS, base64 images)
     PROGMEM_TEMPLATE,  // Nested template in PROGMEM
     RAM_DATA,           // Dynamic RAM data via getter functions
+    DYNAMIC_DATA,       // Dynamic RAM data via getter + userData, rendered as raw bytes
     DYNAMIC_TEMPLATE,
     CONDITIONAL,
     ITERATOR
@@ -64,6 +65,12 @@ typedef void* (*IteratorOpenHandler)(void* userData);
 typedef IteratorStepResult (*IteratorNextHandler)(void* handle, struct IteratorItemView& view);
 typedef void (*IteratorCloseHandler)(void* handle);
 
+struct DynamicDataDescriptor {
+    DynamicTemplateGetter getter;
+    DynamicTemplateLengthGetter getLength;
+    void* userData;
+};
+
 struct DynamicTemplateDescriptor {
     DynamicTemplateGetter getter;
     DynamicTemplateLengthGetter getLength;