basic ECS spawner

addons/gecs/docs/SERIALIZATION.md

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+# GECS Serialization
+
+The GECS framework provides a robust serialization system using Godot's native resource format, enabling persistent game states, save systems, and level data management.
+
+## Quick Start
+
+### Basic Save/Load
+
+```gdscript
+# Save entities with persistent components
+var query = ECS.world.query.with_all([C_Persistent])
+var data = ECS.serialize(query)
+ECS.save(data, "user://savegame.tres")
+
+# Load entities back
+var entities = ECS.deserialize("user://savegame.tres")
+for entity in entities:
+    ECS.world.add_entity(entity)
+```
+
+### Binary Format
+
+```gdscript
+# Save as binary for production (smaller files)
+ECS.save(data, "user://savegame.tres", true)  # Creates .res file
+
+# Load auto-detects format (tries .res first, then .tres)
+var entities = ECS.deserialize("user://savegame.tres")
+```
+
+## API Reference
+
+### ECS.serialize(query: QueryBuilder) -> GecsData
+
+Converts entities matching a query into serializable data.
+
+**Example:**
+
+```gdscript
+# Serialize specific entities
+var player_query = ECS.world.query.with_all([C_Player, C_Health])
+var save_data = ECS.serialize(player_query)
+```
+
+### ECS.save(data: GecsData, filepath: String, binary: bool = false) -> bool
+
+Saves data to disk. Returns `true` on success.
+
+**Parameters:**
+
+- `data`: Serialized entity data
+- `filepath`: Save location (use `.tres` extension)
+- `binary`: If `true`, saves as `.res` (smaller, faster loading)
+
+### ECS.deserialize(filepath: String) -> Array[Entity]
+
+Loads entities from file. Returns empty array if file doesn't exist.
+
+**Auto-detection:** Tries binary `.res` first, falls back to text `.tres`.
+
+## Component Serialization
+
+Only `@export` variables are serialized:
+
+```gdscript
+class_name C_PlayerData
+extends Component
+
+@export var health: float = 100.0        # ✅ Saved
+@export var inventory: Array[String]     # ✅ Saved
+@export var position: Vector2            # ✅ Saved
+
+var _cache: Dictionary = {}              # ❌ Not saved
+```
+
+**Supported types:** All Godot built-ins (int, float, String, Vector2/3, Color, Array, Dictionary, etc.)
+
+## Use Cases
+
+### Save Game System
+
+```gdscript
+func save_game(slot: String):
+    var query = ECS.world.query.with_all([C_Persistent])
+    var data = ECS.serialize(query)
+
+    if ECS.save(data, "user://saves/slot_%s.tres" % slot, true):
+        print("Game saved!")
+
+func load_game(slot: String):
+    ECS.world.purge()  # Clear current state
+
+    var entities = ECS.deserialize("user://saves/slot_%s.tres" % slot)
+    for entity in entities:
+        ECS.world.add_entity(entity)
+```
+
+### Level Export/Import
+
+```gdscript
+func export_level():
+    var query = ECS.world.query.with_all([C_LevelObject])
+    var data = ECS.serialize(query)
+    ECS.save(data, "res://levels/level_01.tres")
+
+func load_level(path: String):
+    var entities = ECS.deserialize(path)
+    ECS.world.add_entities(entities)
+```
+
+### Selective Serialization
+
+```gdscript
+# Save only player data
+var player_query = ECS.world.query.with_all([C_Player])
+
+# Save entities in specific area
+var area_query = ECS.world.query.with_group("area_1")
+
+# Save entities with specific components
+var combat_query = ECS.world.query.with_all([C_Health, C_Weapon])
+```
+
+## Data Structure
+
+The system uses two main resource classes:
+
+### GecsData
+
+```gdscript
+class_name GecsData
+extends Resource
+
+@export var version: String = "0.1"
+@export var entities: Array[GecsEntityData] = []
+```
+
+### GecsEntityData
+
+```gdscript
+class_name GecsEntityData
+extends Resource
+
+@export var entity_name: String = ""
+@export var scene_path: String = ""      # For prefab entities
+@export var components: Array[Component] = []
+```
+
+## Error Handling
+
+```gdscript
+# Serialize never fails (returns empty data if no matches)
+var data = ECS.serialize(query)
+
+# Check save success
+if not ECS.save(data, filepath):
+    print("Save failed - check permissions")
+
+# Handle missing files
+var entities = ECS.deserialize(filepath)
+if entities.is_empty():
+    print("No data loaded")
+```
+
+## Performance
+
+- **Memory:** Creates component copies during serialization
+- **Speed:** Binary format ~60% smaller, faster loading than text
+- **Scale:** Tested with 100+ entities, sub-second performance
+
+## Binary vs Text Format
+
+**Text (.tres):**
+
+- Human readable
+- Editor inspectable
+- Version control friendly
+- Development debugging
+
+**Binary (.res):**
+
+- Smaller file size
+- Faster loading
+- Production builds
+- Auto-detection on load
+
+## File Structure Example
+
+```tres
+[gd_resource type="GecsData" format=3]
+
+[sub_resource type="C_Health" id="1"]
+current = 85.0
+maximum = 100.0
+
+[sub_resource type="GecsEntityData" id="2"]
+entity_name = "Player"
+components = [SubResource("1")]
+
+[resource]
+version = "0.1"
+entities = [SubResource("2")]
+```
+
+## Best Practices
+
+1. **Use meaningful filenames:** `player_save.tres`, `level_boss.tres`
+2. **Organize by purpose:** `user://saves/`, `res://levels/`
+3. **Handle missing components gracefully**
+4. **Use binary format for production**
+5. **Version your save data for compatibility**
+6. **Test with empty query results**
+
+## Limitations
+
+- No entity relationships (planned feature)
+- Prefab entities need scene files present
+- External resource references need manual handling