docs: add Phase 3b-4a deferred items spec (serialization, async load, PBR textures)

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

docs/superpowers/specs/2026-03-25-phase3b-4a-deferred.md

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+# Phase 3b-4a Deferred Items Spec
+
+## A. 씬 직렬화 (`voltex_ecs`)
+
+### JSON 직렬화 (`scene.rs` 확장)
+
+- `serialize_scene_json(world, registry) -> String`
+- `deserialize_scene_json(world, json, registry) -> Result<Vec<Entity>, String>`
+- voltex_ecs 내부에 미니 JSON writer + 미니 JSON parser (voltex_renderer 의존 없음)
+
+포맷:
+```json
+{"version":1,"entities":[
+  {"transform":{"position":[0,0,0],"rotation":[0,0,0],"scale":[1,1,1]},
+   "tag":"player","parent":null,"components":{"rigid_body":{...}}}
+]}
+```
+
+### 바이너리 씬 포맷 (`binary_scene.rs` 신규)
+
+- `serialize_scene_binary(world, registry) -> Vec<u8>`
+- `deserialize_scene_binary(world, data, registry) -> Result<Vec<Entity>, String>`
+
+포맷:
+```
+[4] magic "VSCN"
+[4] version u32 LE
+[4] entity_count u32 LE
+per entity:
+  [4] component_count u32 LE
+  per component:
+    [2] name_len u16 LE
+    [N] name bytes (UTF-8)
+    [4] data_len u32 LE
+    [N] data bytes
+```
+
+### 임의 컴포넌트 등록 (`component_registry.rs` 신규)
+
+```rust
+pub type SerializeFn = fn(&World, Entity) -> Option<Vec<u8>>;
+pub type DeserializeFn = fn(&mut World, Entity, &[u8]) -> Result<(), String>;
+
+pub struct ComponentRegistry {
+    entries: Vec<ComponentEntry>,
+}
+
+struct ComponentEntry {
+    name: String,
+    serialize: SerializeFn,
+    deserialize: DeserializeFn,
+}
+
+impl ComponentRegistry {
+    pub fn new() -> Self
+    pub fn register(&mut self, name: &str, ser: SerializeFn, deser: DeserializeFn)
+    pub fn register_defaults(&mut self)  // Transform, Parent, Tag
+}
+```
+
+---
+
+## B. 비동기 로딩 + 핫 리로드 (`voltex_asset`)
+
+### 비동기 로딩 (`loader.rs` 신규)
+
+```rust
+pub enum LoadState {
+    Loading,
+    Ready,
+    Failed(String),
+}
+
+pub struct AssetLoader {
+    sender: Sender<LoadRequest>,
+    receiver: Receiver<LoadResult>,
+    thread: Option<JoinHandle<()>>,
+    pending: HashMap<u64, PendingAsset>,
+}
+
+impl AssetLoader {
+    pub fn new() -> Self  // 워커 스레드 1개 시작
+    pub fn load<T: Send + 'static>(
+        &mut self, path: PathBuf,
+        parse: fn(&[u8]) -> Result<T, String>,
+    ) -> Handle<T>  // 즉시 핸들 반환, 백그라운드 로딩
+    pub fn state<T: 'static>(&self, handle: Handle<T>) -> LoadState
+    pub fn process_loaded(&mut self, assets: &mut Assets)  // 매 프레임 호출
+    pub fn shutdown(self)
+}
+```
+
+### 핫 리로드 (`watcher.rs` 신규)
+
+```rust
+pub struct FileWatcher {
+    watched: HashMap<PathBuf, SystemTime>,  // path → last_modified
+    poll_interval: Duration,
+    last_poll: Instant,
+}
+
+impl FileWatcher {
+    pub fn new(poll_interval: Duration) -> Self
+    pub fn watch(&mut self, path: PathBuf)
+    pub fn unwatch(&mut self, path: &Path)
+    pub fn poll_changes(&mut self) -> Vec<PathBuf>  // std::fs::metadata 기반
+}
+```
+
+- 외부 크레이트 없음, `std::fs::metadata().modified()` 사용
+- 변경 감지 시 AssetLoader로 재로딩 트리거
+- 기존 Handle은 유지, AssetStorage에서 in-place swap (generation 유지)
+
+---
+
+## C. PBR 텍스처 맵 (`voltex_renderer`)
+
+### 텍스처 바인딩 확장
+
+Group 1 확장 (4→8 바인딩):
+```
+Binding 0-1: Albedo texture + sampler (기존)
+Binding 2-3: Normal map texture + sampler (기존)
+Binding 4-5: Metallic/Roughness/AO (ORM) texture + sampler (신규)
+Binding 6-7: Emissive texture + sampler (신규)
+```
+
+- ORM 텍스처: R=AO, G=Roughness, B=Metallic (glTF ORM 패턴)
+- 텍스처 없으면 기본 1x1 white 사용
+
+### 셰이더 변경
+
+`pbr_shader.wgsl` + `deferred_gbuffer.wgsl`:
+```wgsl
+@group(1) @binding(4) var t_orm: texture_2d<f32>;
+@group(1) @binding(5) var s_orm: sampler;
+@group(1) @binding(6) var t_emissive: texture_2d<f32>;
+@group(1) @binding(7) var s_emissive: sampler;
+
+// Fragment:
+let orm = textureSample(t_orm, s_orm, in.uv);
+let ao = orm.r * material.ao;
+let roughness = orm.g * material.roughness;
+let metallic = orm.b * material.metallic;
+let emissive = textureSample(t_emissive, s_emissive, in.uv).rgb;
+// ... add emissive to final color
+```
+
+### MaterialUniform 변경 없음
+- 기존 metallic/roughness/ao 값은 텍스처 값의 승수(multiplier)로 작동
+- 텍스처 없을 때 white(1,1,1) × material 값 = 기존과 동일 결과
+
+### 텍스처 유틸 확장 (`texture.rs`)
+- `pbr_full_texture_bind_group_layout()` — 8 바인딩 레이아웃
+- `create_pbr_full_texture_bind_group()` — albedo + normal + ORM + emissive
+- `black_1x1()` — emissive 기본값 (검정 = 발광 없음)