Add named project snapshots

docs/project-history-and-external-model-architecture.md

@@ -75,6 +75,20 @@ Project history must use logical project data, not copies of the SQLite database
 
 Large snapshots may later be compressed or stored in object storage, with metadata and checksums retained in the database.
 
+Implemented named-snapshot foundation:
+
+- `project_snapshots` stores project ownership, source revision, schema
+  version, name, optional description, logical payload, SHA-256 and creation
+  metadata
+- snapshot creation requires the expected current revision and reads plus
+  stores the complete project state in one SQLite transaction
+- the versioned payload contains project settings, distribution boards,
+  circuit lists, sections, circuits with device rows, project devices, floors
+  and rooms while excluding global and upgrade-only data
+- snapshot creation/listing does not create a project revision or affect
+  undo/redo eligibility
+- public Create/List endpoints expose metadata but not the logical payload
+
 ## Revit / CSV / IFCGUID Round-Trip Direction
 
 External model exchange should use explicit staging and link entities instead of writing imported rows directly into circuits.
@@ -229,7 +243,7 @@ Completed foundation:
 
 Remaining constraints before completing project version history:
 
-- add a browsable timeline UI plus named logical snapshots and restore
+- add a browsable timeline/snapshot UI, periodic snapshot policy and restore
 - extend project-scoped commands only when further project mutations are
   deliberately added to history; the generic versioned envelope is complete
 - do not model IFCGUID as an overloaded circuit equipment identifier