Make the POSIX timeout module respect target selection during Windows corpus checks

[LunaStev]

Observed at PR #631 head d811e0c2accc8997eecb5071f8752f88e9fb44e4. Failing CI job.

The Windows GNU amd64 job fails its standard-library corpus check with E3001: module 'std::sys::socket' has no symbol 'MSG_DONTWAIT'. The corpus result is 279 passed / 1 failed; all 26 standard-library examples pass.

The new std/net/posix_timeout.wave imports the Unix-only symbol unconditionally. Target selection at the tcp.wave import site does not protect this file when the corpus checker checks each module directly. Locally reproduced with:

target/debug/wavec check std/net/posix_timeout.wave --target x86_64-pc-windows-gnu

Scope: make the module's imports/declarations target-appropriate, or apply an explicit supported-target contract in corpus discovery. Do not invent a Windows MSG_DONTWAIT constant or drop platform validation wholesale.

Acceptance:

• Windows GNU corpus and examples pass, and the native Windows timeout implementation remains selected.
• Linux/macOS/FreeBSD still check the real POSIX timeout implementation.
• Add a focused regression for standalone module checking on an incompatible target.

This is a target-selection regression introduced in #631, separate from the runtime deadline behavior in #510.

Source references:

• std/net/posix_timeout.wave
• std/net/tcp.wave
• tools/check_wave_corpus.py