feat: convert windows to fenestration surfaces

[hongyuanjia]

Summary

Closes #3.

This PR is stacked on #9 so the window conversion can be reviewed against the check-clean conversion baseline. After #9 lands, this branch can be rebased back onto main.

Changes

• Implement WINDOW to FenestrationSurface:Detailed conversion.
• Choose the room-side parent surface from MAIN_ENCLOSURE/SURFACE and keep vertex order aligned with parent wall side.
• Include window conversion in the default to_eplus() path when WINDOW records exist.
• Add the two-layer test shape: an in-memory SQLite test plus a skip_on_cran() real DeST SQLite fixture test.

Local checks run with DESTEP_TEST_FIXTURE_DIR pointing at the real fixture directory: focused conv-window/to_eplus tests passed, and devtools::check returned 0 errors, 0 warnings, and 1 environment note for future file timestamp verification.

Out of scope

• Window shading and detailed DeST window-type optical mappings remain separate follow-up work.