🤖 Fix proofreading issues in bevy skills

- Fix `ref` keyword used as variable name in code examples
- Fix "retries next tick" → systems continue on normal schedule
- Fix `State::set()` → `NextState::set()` in upgrade skill
- Fix `SceneRoot` → `WorldAssetRoot` for 0.19 consistency
- Fix Replace lifecycle event description (overwritten, not removed)
- Fix grep patterns (quoting, trailing spaces)
- Remove third-party physics/networking content (not Bevy built-ins)

plugins/bevy/skills/bevy-upgrade/references/0.18-to-0.19.md

@@ -14,7 +14,7 @@ Smaller than 0.17→0.18 but with one deep semantic change: resources became com
 ## API shape changes
 
 - **`Assets::get_mut`** returns `AssetMut<A>` (mutation-tracked), not `&mut A`. Deref for access; code storing the `&mut A` needs restructuring.
-- **`Ref<T>.clone()`** returns `Ref<T>`, not the inner value. Old code relying on clone-through must use `ref.deref().clone()`.
+- **`Ref<T>.clone()`** returns `Ref<T>`, not the inner value. Old code relying on clone-through must use `val.deref().clone()`.
 - **GLTF material loading** returns `GltfMaterial`; request a `StandardMaterial` with the `#Material0/std` label suffix.
 - **`EntityComponentsTrigger`** gained archetype fields — exhaustive destructuring breaks; add `..`:
   `let EntityComponentsTrigger { components, .. } = e.trigger();`
@@ -28,9 +28,9 @@ Smaller than 0.17→0.18 but with one deep semantic change: resources became com
 
 ## Porting checklist
 
-1. `grep -rn "SceneRoot\|Scene>" ` — rename to `WorldAssetRoot`/`WorldAsset`.
+1. `grep -rn "SceneRoot\|Scene>"` — rename to `WorldAssetRoot`/`WorldAsset`.
 2. `grep -rn "Query<Entity[,>]"` and other broad queries — add `Without<IsResource>` where resources must not appear.
 3. `grep -rn "derive(Component" | grep "Resource"` — find dual-derive types; split them.
-4. `grep -rn "get_mut" ` on `Assets<...>` — adapt to `AssetMut`.
+4. `grep -rn "get_mut"` on `Assets<...>` — adapt to `AssetMut`.
 5. `grep -rn "EntityComponentsTrigger {"` — add `..` to destructuring.
 6. GLTF material handles — append `/std` labels where `StandardMaterial` is expected.