fix: refactor proxy header logic into ConnectionStream

[junderw]

1. Large refactor. Moved the Proxy parse logic into ConnectionStream (the wrapper around TcpStream or UnixStream)
2. Should make it easier to nab the proxy IP without blocking, before we start logging stuff.

NGINX sends the Proxy header as soon as the client finishes the TCP connection regardless of whether the client sends anything yet... so this should work out fine.

Client                    NGINX                     Origin
  |                         |                        |
  |---- TCP SYN ----------->|                        |
  |<--- SYN/ACK ------------|                        |
  |---- ACK --------------->|                        |
  |                         |---- TCP SYN ---------->|
  |                         |<--- SYN/ACK -----------|
  |                         |---- ACK -------------->|
  |                         |---- PROXY header ----->|
  |                         |     (immediately)      |
  |                         |                        |
  |                         |                        |
  |                         |                        |
  |---- First payload ----->|---- First payload ---->|

[Copilot]

Pull request overview

This PR refactors PROXY-protocol (HAProxy) header parsing so it lives inside ConnectionStream, enabling early/non-blocking resolution of the proxied client address for logging and per-client connection limiting.

Changes:

• Introduces ConnectionStream buffering (leftover) and one-shot PROXY probing (resolve_proxy) to strip headers before request parsing.
• Updates connection lifecycle/logging to use an eagerly resolved client identifier.
• Simplifies the request reader by removing the prior pre-read/Cursor chaining logic.

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