Add modular prompt system with clean architecture

Introduces a scalable, production-grade prompt composition system for ComfyUI. Adds pure data libraries, schema-driven rules, a content-agnostic engine, and minimal node interface. Includes documentation (ARCHITECTURE.md, MIGRATION.md, QUICKSTART.md, README.md), three example libraries, and schema configuration for block order, weights, and model profiles.

engine/composer.py

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+"""
+Pure composition engine - content agnostic
+"""
+import json
+import os
+import glob
+
+class PromptComposer:
+    """
+    Generic prompt composition engine.
+    Knows nothing about content, only follows schema rules.
+    """
+    
+    def __init__(self, schema_path, libraries_path):
+        self.schema = self._load_json(schema_path)
+        self.libraries = self._load_libraries(libraries_path)
+        
+    def _load_json(self, path):
+        with open(path, 'r', encoding='utf-8') as f:
+            return json.load(f)
+    
+    def _load_libraries(self, path):
+        libraries = {}
+        for json_file in glob.glob(os.path.join(path, "*.json")):
+            lib_id = os.path.splitext(os.path.basename(json_file))[0]
+            libraries[lib_id] = self._load_json(json_file)
+        return libraries
+    
+    def get_library_names(self):
+        return list(self.libraries.keys())
+    
+    def get_block_options(self, library_id, block_name):
+        if library_id not in self.libraries:
+            return []
+        blocks = self.libraries[library_id].get("blocks", {})
+        return list(blocks.get(block_name, {}).keys())
+    
+    def apply_weight(self, text, weight, model_profile):
+        model_config = self.schema["models"].get(model_profile, {})
+        if not model_config.get("enable_weights", False):
+            return text
+        if weight == 1.0:
+            return text
+        
+        adjusted_weight = weight * model_config.get("weight_multiplier", 1.0)
+        weight_format = model_config.get("weight_format", "({text}:{weight})")
+        
+        return weight_format.format(text=text, weight=f"{adjusted_weight:.1f}")
+    
+    def compose(self, library_id, selections, model_profile="sdxl"):
+        """
+        Compose final prompt from selections.
+        
+        Args:
+            library_id: Selected library
+            selections: Dict of {block_name: selected_option}
+            model_profile: "sdxl" or "flux"
+        
+        Returns:
+            Final composed prompt string
+        """
+        library = self.libraries.get(library_id, {})
+        blocks = library.get("blocks", {})
+        
+        structure = self.schema["structure"]
+        weights = self.schema["weights"]
+        
+        parts = []
+        block_order = structure["block_order"]
+        separator = structure["separator"]
+        break_positions = structure.get("break_positions", [])
+        
+        for i, block_name in enumerate(block_order):
+            selected_option = selections.get(block_name)
+            if not selected_option:
+                continue
+            
+            block_data = blocks.get(block_name, {})
+            text = block_data.get(selected_option, "")
+            
+            if not text:
+                continue
+            
+            weight = weights.get(block_name, 1.0)
+            weighted_text = self.apply_weight(text, weight, model_profile)
+            
+            parts.append(weighted_text)
+            
+            if (i + 1) in break_positions:
+                parts.append("BREAK")
+        
+        return separator.join(parts)