feature/state arc builder

[uemuradevexe]

Summary

Changes:

• AppState::with_arc<T: ?Sized>() — stores Arc directly in the state map under TypeId::of::<Arc>(), no newtype needed
• Rapina::state_arc() — builder method exposing with_arc on the public app API
• State<Arc> extractor works without modification — deref chain gives direct access to trait methods
• State extractor error message updated to mention .state_arc()

Tests added:

• state.rs: concrete type, trait object, coexistence with .with(), missing state, overwrite
• extract/mod.rs: end-to-end extraction of Arc via FromRequestParts

Related Issues

Closes #495

Checklist

• cargo fmt passes
• cargo clippy has no warnings
• Tests pass
• Documentation updated (if needed)

[ShiraiEd]

It might be clearer written as Arc::clone(&*state) instead of (*state).clone() since that makes the intent explicit

[ShiraiEd]

Great work!

The feature is still missing in the dependency injection docs, worth mentioning there.