feat(proxy): add Windows WinINet backend

[kev1n77]

Summary

• persist backend-specific raw proxy snapshots instead of reducing originals to a generic mode
• add a Windows WinINet backend for HKCU Internet Settings with exact registry value type/byte restoration
• validate manual/PAC endpoints before mutation and broadcast WinINet refresh notifications
• add isolated HKCU round-trip tests and a Windows-runner real proxy force-kill experiment
• keep SPIKE-003 open until the macOS backend is verified

Local verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings

The real WinINet mutation test is intentionally ignored locally and enabled only by the dedicated Windows CI step. Progress documentation will be updated only after that experiment passes.