feat(ecs): add JSON/binary scene serialization and component registry

- Mini JSON writer/parser in voltex_ecs (no renderer dependency)
- ComponentRegistry with register/find/register_defaults
- serialize_scene_json/deserialize_scene_json with hex-encoded components
- serialize_scene_binary/deserialize_scene_binary (VSCN binary format)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/voltex_ecs/src/component_registry.rs

@@ -0,0 +1,198 @@
+/// Registration-based component serialization for scene formats.
+/// Each registered component type has a name, a serialize function,
+/// and a deserialize function.
+
+use crate::entity::Entity;
+use crate::world::World;
+
+pub type SerializeFn = fn(&World, Entity) -> Option<Vec<u8>>;
+pub type DeserializeFn = fn(&mut World, Entity, &[u8]) -> Result<(), String>;
+
+pub struct ComponentEntry {
+    pub name: String,
+    pub serialize: SerializeFn,
+    pub deserialize: DeserializeFn,
+}
+
+pub struct ComponentRegistry {
+    entries: Vec<ComponentEntry>,
+}
+
+impl ComponentRegistry {
+    pub fn new() -> Self {
+        Self { entries: Vec::new() }
+    }
+
+    pub fn register(&mut self, name: &str, ser: SerializeFn, deser: DeserializeFn) {
+        self.entries.push(ComponentEntry {
+            name: name.to_string(),
+            serialize: ser,
+            deserialize: deser,
+        });
+    }
+
+    pub fn find(&self, name: &str) -> Option<&ComponentEntry> {
+        self.entries.iter().find(|e| e.name == name)
+    }
+
+    pub fn entries(&self) -> &[ComponentEntry] {
+        &self.entries
+    }
+
+    /// Register the default built-in component types: transform and tag.
+    pub fn register_defaults(&mut self) {
+        self.register("transform", serialize_transform, deserialize_transform);
+        self.register("tag", serialize_tag, deserialize_tag);
+    }
+}
+
+impl Default for ComponentRegistry {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// ── Transform: 9 f32s in little-endian (pos.xyz, rot.xyz, scale.xyz) ─
+
+fn serialize_transform(world: &World, entity: Entity) -> Option<Vec<u8>> {
+    let t = world.get::<crate::Transform>(entity)?;
+    let mut data = Vec::with_capacity(36);
+    for &v in &[
+        t.position.x, t.position.y, t.position.z,
+        t.rotation.x, t.rotation.y, t.rotation.z,
+        t.scale.x, t.scale.y, t.scale.z,
+    ] {
+        data.extend_from_slice(&v.to_le_bytes());
+    }
+    Some(data)
+}
+
+fn deserialize_transform(world: &mut World, entity: Entity, data: &[u8]) -> Result<(), String> {
+    if data.len() < 36 {
+        return Err("Transform data too short".into());
+    }
+    let f = |off: usize| f32::from_le_bytes([data[off], data[off + 1], data[off + 2], data[off + 3]]);
+    let t = crate::Transform {
+        position: voltex_math::Vec3::new(f(0), f(4), f(8)),
+        rotation: voltex_math::Vec3::new(f(12), f(16), f(20)),
+        scale: voltex_math::Vec3::new(f(24), f(28), f(32)),
+    };
+    world.add(entity, t);
+    Ok(())
+}
+
+// ── Tag: UTF-8 string bytes ─────────────────────────────────────────
+
+fn serialize_tag(world: &World, entity: Entity) -> Option<Vec<u8>> {
+    let tag = world.get::<crate::scene::Tag>(entity)?;
+    Some(tag.0.as_bytes().to_vec())
+}
+
+fn deserialize_tag(world: &mut World, entity: Entity, data: &[u8]) -> Result<(), String> {
+    let s = std::str::from_utf8(data).map_err(|_| "Invalid UTF-8 in tag".to_string())?;
+    world.add(entity, crate::scene::Tag(s.to_string()));
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{World, Transform};
+    use voltex_math::Vec3;
+
+    #[test]
+    fn test_register_and_find() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        assert!(registry.find("transform").is_some());
+        assert!(registry.find("tag").is_some());
+        assert!(registry.find("nonexistent").is_none());
+    }
+
+    #[test]
+    fn test_entries_count() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        assert_eq!(registry.entries().len(), 2);
+    }
+
+    #[test]
+    fn test_serialize_transform() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        let mut world = World::new();
+        let e = world.spawn();
+        world.add(e, Transform::from_position(Vec3::new(1.0, 2.0, 3.0)));
+
+        let entry = registry.find("transform").unwrap();
+        let data = (entry.serialize)(&world, e);
+        assert!(data.is_some());
+        assert_eq!(data.unwrap().len(), 36); // 9 f32s * 4 bytes
+    }
+
+    #[test]
+    fn test_serialize_missing_component() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        let mut world = World::new();
+        let e = world.spawn();
+        // no Transform added
+
+        let entry = registry.find("transform").unwrap();
+        assert!((entry.serialize)(&world, e).is_none());
+    }
+
+    #[test]
+    fn test_roundtrip_transform() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        let mut world = World::new();
+        let e = world.spawn();
+        world.add(e, Transform {
+            position: Vec3::new(1.0, 2.0, 3.0),
+            rotation: Vec3::new(0.1, 0.2, 0.3),
+            scale: Vec3::new(4.0, 5.0, 6.0),
+        });
+
+        let entry = registry.find("transform").unwrap();
+        let data = (entry.serialize)(&world, e).unwrap();
+
+        let mut world2 = World::new();
+        let e2 = world2.spawn();
+        (entry.deserialize)(&mut world2, e2, &data).unwrap();
+
+        let t = world2.get::<Transform>(e2).unwrap();
+        assert!((t.position.x - 1.0).abs() < 1e-6);
+        assert!((t.position.y - 2.0).abs() < 1e-6);
+        assert!((t.position.z - 3.0).abs() < 1e-6);
+        assert!((t.rotation.x - 0.1).abs() < 1e-6);
+        assert!((t.scale.x - 4.0).abs() < 1e-6);
+    }
+
+    #[test]
+    fn test_roundtrip_tag() {
+        let mut registry = ComponentRegistry::new();
+        registry.register_defaults();
+        let mut world = World::new();
+        let e = world.spawn();
+        world.add(e, crate::scene::Tag("hello world".to_string()));
+
+        let entry = registry.find("tag").unwrap();
+        let data = (entry.serialize)(&world, e).unwrap();
+
+        let mut world2 = World::new();
+        let e2 = world2.spawn();
+        (entry.deserialize)(&mut world2, e2, &data).unwrap();
+
+        let tag = world2.get::<crate::scene::Tag>(e2).unwrap();
+        assert_eq!(tag.0, "hello world");
+    }
+
+    #[test]
+    fn test_deserialize_transform_too_short() {
+        let mut world = World::new();
+        let e = world.spawn();
+        let result = deserialize_transform(&mut world, e, &[0u8; 10]);
+        assert!(result.is_err());
+    }
+}