Convert the Writer's operation vocabulary to a closed enum

WriteOperation replaces free-form operation strings throughout
BoardWriter, per the settled rule in 02 § Write-failure surfacing: the
banner layer will switch exhaustively over it, so a new operation
without a banner rendering is a compile-time hole. Titles enrich at the
two points a document read makes them known (updateIndex, and the
move/copy pre-flight), so failures after the read name the item;
purge never reads and stays title-less. Free-form English survives
only in the diagnostic reason.

Full suite 300 tests in 56 suites green.

Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A

KanbanTests/BoardStoreTests.swift

@@ -471,7 +471,7 @@ struct BoardStoreTests {
 
         // A Writer operation that fails partway has still touched disk, so the bracket has to close
         // on the throwing path too — an unbalanced one would suspend the watcher for the session.
-        let boom = BoardWriteError(operation: "probe", path: "/nowhere", reason: .io(message: "disk full"))
+        let boom = BoardWriteError(operation: .style(title: nil), path: "/nowhere", reason: .io(message: "disk full"))
         do {
             try store.performWrite { () throws(BoardWriteError) -> Void in throw boom }
             Issue.record("expected the operation's own error to propagate")