Add lazy chat turn paging

[connor4312]

Adds lazy loading for old turns in chat state.

Summary:

• Add SubscribeParams.view.turns?: number as advisory initial chat turn windowing.
• Add ChatState.turnsNextCursor and server-only chat/turnsLoaded for paging older turns into reducer state.
• Rework fetchTurns to accept cursor and rely on chat/turnsLoaded before responding.
• Update TypeScript, Rust, Kotlin, Swift, and Go generated types plus handwritten reducers/helpers.
• Remove 0.5.0 from generated supported protocol versions because the fetchTurns contract changes non-additively.

Fixes #272

Validation:

• npm run generate
• cargo fmt
• cargo check
• cargo test
• npm run test
• TypeScript client npm test
• Go go test ./...
• Swift swift test
• Kotlin via Podman ./gradlew test --no-daemon --console=plain
• git diff --check