Generate desktop-win dry-run package plans

[RasputinKaiser]

Fixes #169.

Summary

• make desktop-win dry-run builds write a Windows package-plan JSON artifact
• include planned MSIX/MSI artifacts and makeappx/WiX/signtool command steps without executing Windows-only tooling
• mark missing signing certificate thumbprints as explicit follow-up data in the plan
• add focused coverage for the both distribution output and missing-signing-cert handling

Verification

• corepack pnpm vitest run packages/targets/desktop-win/src/index.test.ts
• corepack pnpm --filter @profullstack/sh1pt-target-desktop-win typecheck
• corepack pnpm --filter @profullstack/sh1pt-target-desktop-win build
• git diff --check