Problem

main is still red after #6306. That PR gated the scriptable transform_tool_result hook tests behind #[cfg(unix)] (including make_transform_engine), but missed one import: use std::sync::Arc in the same test module is now referenced only by the Unix-gated make_transform_engine (Arc::new(MemorySubstrate::open_in_memory(..))). The rest of the module uses the fully-qualified std::sync::Arc (e.g. StubDriver's driver()), so on Windows the short import is unused and the build fails:

error: unused import: `std::sync::Arc`
  --> crates/librefang-runtime/src/context_engine/scriptable/engine.rs:1254
  = note: `-D unused-imports` implied by `-D warnings`

Both Windows shards fail at the cargo test --no-run compile step, so main's post-merge CI stayed red and every PR keeps inheriting it.

Fix

Gate use std::sync::Arc behind #[cfg(unix)], matching the already-gated make_transform_engine / MemorySubstrate / ToolExecutionStatus. Verified Arc (short name) is used only at the single gated site; everything else in the module uses std::sync::Arc fully-qualified.

Verification

Linux / macOS are unaffected — cfg(unix) is true there, so Arc stays imported and used. Windows is the platform that was failing and can't be reproduced locally (no native Rust toolchain / Docker this session), so the Windows lane on main after merge is the authoritative check (PR CI skips Windows by design). The fix is the exact, minimal counterpart of the compile error CI reported.

Completes #6304. Follow-up to #6306.