fix: preserve storey height in zone conversion

[hongyuanjia]

Summary

Closes #4

Changes

• Select STOREY.HEIGHT when reading ROOM/STOREY data for Zone conversion.
• Add a focused in-memory SQLite test for the narrow conversion logic.
• Add a skip_on_cran() real DeST SQLite fixture test that verifies converted Zone Ceiling Height values match source storey heights.

Out of scope

• No changes to surface, construction, or schedule conversion.

[hongyuanjia]

Superseded by #9, which includes this zone-height fix together with the full local devtools::check gate.