fix(11-05): sync single-statement and plain-transaction writes, type engine status errors

- pin the sync callbacks before gorm:commit_or_rollback_transaction: an
  After-only anchor is appended past the commit and lagoon's after-commit
  flush, so single-statement writes never synced
- give the gate and the document builder a clean session: Session with NewDB
  and a Context clones the write's statement, and a later WithContext queried
  through the written model's table
- typesense.StatusError carries method, path and status, never the body
- README: sync semantics, the three gates, delete on soft delete, and the
  SQL re-gate required of SearchIDs callers

modules/beachcomber/searchable.go

@@ -74,7 +74,9 @@ type Query struct {
 }
 
 // Gate is the application kill-switch consulted before every sync. It
-// reads its setting through db; any error must count as off.
+// reads its setting through db, a clean session on the write's connection
+// (inside a caller's plain transaction, a savepoint of it); any error must
+// count as off.
 type Gate interface {
 	Enabled(ctx context.Context, db *gorm.DB) bool
 }