feat: support passing proxy_protocol to reality backend

[aJC7737]

TL;DR: This change adds proxy-protocol support to Reality inbound so backends (for example NGINX) can see the original client source address.

What This PR Adds

1. New Reality inbound option: proxy-protocol

   • 0: disabled (default)
   • 1: send PROXY protocol v1 header
   • 2: send PROXY protocol v2 header

2. PROXY header forwarding for Reality backend connections

   • Adds ProxyProtocol to Reality runtime config.
   • Validates allowed values (0/1/2).
   • When enabled, writes the PROXY header immediately after dialing the Reality backend and before normal payload forwarding.

3. Source/destination address propagation

   • Captures inbound connection remote/local TCP addresses.
   • Passes them through context so generated PROXY headers contain:
     • original client source address/port
     • inbound listener destination address/port

4. PROXY protocol encoding support

   • Implements dedicated builders for:
     • PROXY v1 (text format)
     • PROXY v2 (binary format)

5. Documentation updates

   • Adds reality-config.proxy-protocol examples in config documentation.

Tests

Added/updated unit tests covering:

• disabled mode (proxy-protocol = 0, no header emitted)
• v1 header generation/writing
• v2 header generation/writing
• missing address context error path
• invalid version error path

Scope

• Inbound Reality listener path only.
• No outbound behavior changes.