Fix intermittent Windows Harness workspace crash

[wisdomqin]

What changed

• Pin Koffi to 3.1.5 so packaged Windows builds use the stable native Windows binaries.
• Extend the packaged Windows Harness smoke test to create a Unicode-path workspace and a session, then verify the process remains alive.
• Update the isolated Windows preview release description for this validation path.

Why

An affected Windows x64 machine intermittently exited with native access violation 0xC0000005 immediately after the Harness entry loaded. The folder picker itself returned successfully; the failure happened during subsequent Harness startup or workspace/session initialization. Koffi 3.1.5 restores native Windows compilation for stability.

Validation

• npm test — 79 tests passed
• npm run typecheck
• git diff --check
• Three-platform preview CI passed
• Packaged Windows smoke test created a Chinese-path workspace and session and observed the Harness for 20 seconds
• Affected Windows machine was retested several times successfully with desktop-preview-20260817.2

Preview: https://github.com/dataelement/dsh-desktop/releases/tag/desktop-preview-20260817.2