feat(p2p-09): add data-plane redirect plumbing (#671)

[jlon]

Summary

• Introduce control-plane redirects and ticketed follow-up fetches for the P2P data plane.
• Keep this slice intermediate-only; rollout-safe redirect behavior still depends on later correctness and lifecycle hardening.

Design

• Redirect only when the first chunk is serviceable and preserve direct worker fallback.
• Isolate redirect/ticket plumbing from later correctness and performance work to keep review scope narrow.

Implementation

• Add redirect responses, data-plane listener/bind lifecycle, and framing for redirected fetches.
• Wire block follow-up reads and the fetched-cache visibility required for redirected fetches.

Verification

• Verify redirects only occur when the first chunk can be served.
• Verify follow-up responses avoid useless tickets and worker fallback remains available.

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