Add public building project setting

docs/spec/08-current-product-backlog.md

@@ -54,7 +54,7 @@ requirements and intended sequencing, not proof of implementation.
 
 The pre-release cleanup established the following compatibility boundary:
 
-- [x] Pre-release databases, portable JSON exports, logical snapshots and
+- [x] Pre-baseline databases, portable JSON exports, logical snapshots and
   persisted command histories are explicitly unsupported.
 - [x] Migration `0000` builds the complete supported schema on an empty
   database.
@@ -109,9 +109,16 @@ The pre-release cleanup established the following compatibility boundary:
   device rows with a valid phase are three-phase.
 - [x] Changing project voltage settings updates all project devices and circuits
   atomically in the same persistent Undo/Redo step.
-- [x] Baseline migration `0000` and project-state schema version `1` store only
-  the canonical derived values. Pre-release imports are intentionally
-  unsupported.
+- [x] Baseline migration `0000` stores only canonical derived values.
+  Pre-baseline imports are intentionally unsupported.
+
+## Building Classification
+
+- [x] Store whether a project represents a public building.
+- [x] Edit the value in the project settings and preserve it through persistent
+  Undo/Redo, snapshots and portable project transfers.
+- [x] Provide the value as an explicit future sizing input for selecting
+  halogen-free cables and lines; automatic sizing remains deferred.
 
 ## Circuit List Power Summary