feat: add timed TCP connection establishment

[zuudevs]

Summary

Adds the v1 TCP connection-establishment layer on top of the merged resolver and native-socket primitives.

Added

• src/platform/socket_connect.hpp / .cpp
  • native socket creation helper
  • non-blocking connect setup
  • Windows select() / POSIX poll() wait path
  • SO_ERROR verification after readiness
  • restore blocking mode after a successful connect
  • platform-native diagnostic code capture
• src/net/tcp_connection.hpp / .cpp
  • move-only internal TcpConnection
  • candidate iteration in resolver order
  • one total connect-timeout budget shared across all endpoint candidates
  • fallback to later candidates after socket-creation/connect failure
  • explicit SocketCreateFailed, ConnectFailed, and ConnectTimeout mapping
  • deterministic RAII close semantics through NativeSocket
• tests/tcp_connection_test.cpp
  • real IPv4 loopback listener connection
  • candidate fallback
  • zero-timeout behavior
  • empty-candidate behavior
  • explicit close semantics

Build changes

• compile timed-connect and TCP connection sources into the internal target
• wire TCP connection tests into the existing GoogleTest target

Timeout semantics

The configured connect timeout is a budget for the complete connection operation, not a fresh timeout for every resolved address. IPv6/IPv4 candidate fallback therefore remains bounded by the caller's requested timeout.

CI / determinism

Connection tests use a local ephemeral loopback listener only. They do not require public internet or external services.

Scope

This PR intentionally does not add request writes, response reads, partial-I/O loops, read/write timeouts, HTTP serialization, or connection reuse policy. Those build on this established connection primitive in follow-up PRs.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Retry poll when a signal interrupts the wait

On POSIX, if a signal is delivered while a connection attempt is pending, poll() returns -1 with errno == EINTR; this path reports ConnectFailed and abandons a potentially viable endpoint well before the configured deadline. Retry the wait after recomputing the remaining timeout when the error is EINTR, while preserving other errors as failures.

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