feat: native SQLite KV channel

[NathanFlurry]

Summary

• Implement native SQLite addon (@rivetkit/sqlite-native) using napi-rs with a custom VFS backed by KV over WebSocket
• Add KV channel protocol (BARE schema + TypeScript/Rust implementations) for direct SQLite-to-engine KV communication
• Add engine KV channel WebSocket server (pegboard-kv-channel crate) with single-writer lock enforcement
• Integrate KV channel into manager router, guard routing, and file-system/cloudflare-workers drivers
• Align WASM VFS with native VFS: add deleteRange, PRAGMA settings, matching chunk layout and key encoding
• Add per-actor channel routing, unconditional lock eviction on reconnect, and spawn_blocking for napi exports
• Add Rust VFS unit tests, integration tests, and review findings documentation

Test plan

• Run pnpm test driver-file-system-native-sqlite from rivetkit-typescript/packages/rivetkit
• Run pnpm test driver-file-system to verify WASM VFS still passes
• Run cargo test -p rivetkit-sqlite-native for Rust unit/integration tests
• Run cargo check on the full workspace
• Verify KV channel WebSocket endpoint responds on manager router

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