V1.6.0 — Multi-floor stairs & precision update.

 - New: split-level and basement-to-upper-floor stairs — mark any landing as a floor level and get its own Rough Opening calculation
 - Rough Opening sizing now accounts for tread nosing for a more precise result
 - Deck Stair Mode now clearly supports wraparound and corner decks via multi-flight layouts
 - Hardened against extreme and edge-case inputs

 Still 100% offline. No tracking, no accounts, no servers.