add chainsync impl based on pure-stage

[rkuhn]

no individual test yet, will test when new stages are complete

Summary by CodeRabbit

• New Features

  • In‑memory mempool, full transaction‑submission and block‑fetch protocols, handshake and keep‑alive protocols, peer manager, and a new Tokio‑based network connector.

• Improvements

  • Unified tip/point/slot/height handling across components; protocol deserializers centralized; mempool and store effects exposed for integration; richer scheduling, supervision and tracing in the stage runtime.

• Bug Fixes

  • Clearer header decode errors, improved fetch/timeout handling, and updated tests/mocks to match the new protocols and mempool behavior.

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