feat(engine): write-through persistence (M13 D2)

- facade storage session manager: project handles bind to the default
  SQLite library on project_new/load; every undo push/group_end/jump
  write-throughs via DatabaseBackend::save (diff journal); project_free
  flushes and unbinds
- background snapshot thread (Storage/SnapshotIntervalSec, latest-wins,
  newest 3 kept) with exit flush (oakengine_storage_flush)
- config-gated: storage only activates with an explicit
  Storage/Backend=sqlite, so headless consumers and tests never touch
  the real library; new exports: storage_flush/is_bound/last_error
- it_storage: kill -9 recovery, cross-session undo, snapshot pruning,
  multi-project isolation, graceful degradation
- also fixes a real config test polluting the user config.ini and the
  undo-stack test races

Cargo.lock

@@ -4814,12 +4814,15 @@ dependencies = [
  "oaknode",
  "oakplugin",
  "oakrender",
+ "oakstorage",
  "oaktask",
  "oaktimeline",
  "oakundo",
+ "sea-orm",
  "serde",
  "serde_json",
  "thiserror 2.0.20",
+ "tokio",
 ]
 
 [[package]]