🤖 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/0.17-to-0.18.md

@@ -33,7 +33,7 @@ New in 0.18: `#[derive(EntityEvent)]` with `#[event_target]` for entity-targeted
 
 ## API shape changes
 
-- **Fallible systems**: systems may return `Result` (= `Result<(), BevyError>`); errors log and the system retries next tick. Adopt during the bump wherever it deletes `unwrap()`s cheaply; systematically during cleanup.
+- **Fallible systems**: systems may return `Result` (= `Result<(), BevyError>`); errors are logged and the system continues running on its normal schedule. Adopt during the bump wherever it deletes `unwrap()`s cheaply; systematically during cleanup.
 - **Mesh accessors** became `try_*` returning `Result<_, MeshAccessError>`.
 - **`MaterialPlugin` fields** `prepass_enabled` / `shadows_enabled` became `Material` trait methods.
 - **`LoadContext::path()`** returns `AssetPath`, not `Path`; image `reinterpret_*` methods return `Result`.
@@ -52,6 +52,6 @@ New in 0.18: `#[derive(EntityEvent)]` with `#[event_target]` for entity-targeted
 2. `grep -rn "derive(Event)"` — decide per type: buffered (→ `Message`) or observed (stays `Event`, needs a trigger + observer).
 3. `grep -rn "remove_child\|clear_children"` — rename to `detach_*`.
 4. `grep -rn "next_state.set\|NextState"` — audit same-state re-fire.
-5. `grep -rn "@group(2)" --include=*.wgsl` — move material bindings to group 3.
+5. `grep -rn "@group(2)" --include="*.wgsl"` — move material bindings to group 3.
 6. `grep -rn "AmbientLight"` — split into resource vs camera component usage.
 7. Check `Cargo.toml` for `default-features = false` on bevy → add input features.