Fix V4 Windows runtime link contract

[OttoApocalypse69]

What changed

• centralize checker platform runtime-link arguments
• link Winsock on Windows, libm on Linux, and no extra runtime library on macOS/unknown hosts
• apply the same policy to runtime smoke, LLVM runtime primitives, and V3 bootstrap stage0/stage2/final links
• bind platform, include, runtime-link, and final-link policy into bumped compile-cache identities
• add an unconditional fail-closed pure-Python platform/cache contract test

Why

Generated runtime C references Winsock on Windows, but two checker paths omitted -lws2_32; an external wrapper was masking the defect. Cache keys also omitted link policy, allowing stale pre-fix executables to bypass the repaired command.

Local validation

• independent review of a6e7de0060967e920a0fbfb7dcf3d65f225d49b3...b5348a83bc4dec4822892c53a0d2fdef7801367e: APPROVE, no P0-P3
• Python compile/import and direct link-policy self-test
• git diff --check
• serialized real-Clang task return semantic boundary smoke under Global\FreakCheckV4, no wrapper: runtime primitives, V3 stage0/stage2/final link+execute, and task-return native smoke all passed

Pending remote evidence

• Linux/macOS native CI confirmation
• automated review on the current head

Boundaries

This is checker infrastructure only. It does not change the shipping compiler, runtime sources, native object-only links, target semantics, or PR readiness state.

[coderabbitai]

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