refactor(client): replace unsafe libc calls with nix for Linux ancillary data

Eliminate unsafe code blocks and manual pointer arithmetic in Linux-specific receive paths by utilizing nix for socket options and control message parsing. This improves safety and maintainability while preserving the existing kernel RX timestamp and traffic class functionality.

Cargo.lock

@@ -710,7 +710,7 @@ name = "irtt-client"
 version = "0.4.0"
 dependencies = [
  "irtt-proto",
- "libc",
+ "nix 0.31.3",
  "socket2",
  "thiserror 2.0.20",
  "tokio",
@@ -927,6 +927,7 @@ dependencies = [
  "cfg-if",
  "cfg_aliases",
  "libc",
+ "memoffset",
 ]
 
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