Implement Hierarchy Node Editor save and validation flow

Separate the BLoC into two event handlers: _onCreateNode and _onUpdateNode. Both call HierarchyStructureValidator.validate() first with the full in-memory tree snapshot (loaded when the form opens). For the optimistic update pattern: (1) emit an intermediate state with the tentative node added to the local tree, (2) attempt the Supabase write, (3) on success emit SaveSuccess, (4) on failure emit SaveFailure and re-emit the pre-optimistic tree snapshot stored before step 1. Use Supabase's upsert only if the create/edit distinction is not important — here they are distinct, so use insert and update separately to keep error handling clean.

The parent dropdown debounce should be implemented as a BLoC event: ParentSearchQueryChanged(query) — the BLoC handles debounce internally using Stream.debounceTime from rxdart (already likely in the project). Avoid placing debounce logic in the widget layer. For the in-memory tree cycle check, implement as a depth-first traversal starting from the proposed parent and checking if the current node's ID appears as an ancestor.

Unit tests (flutter_test): test HierarchyNodeEditorBloc save flow for create mode — mock Supabase client, assert SaveSuccess emitted on mock success and SaveFailure on mock error; test rollback by asserting tree state reverts after mock write failure; test cycle detection by constructing an in-memory tree with a known cycle and asserting ValidationError emitted before any write. Unit tests for HierarchyStructureValidator: test all cycle cases (direct parent = self, grandparent loop, multi-hop cycle) and all depth-limit violations. Integration tests: run against Supabase test instance — create a node, verify it appears in the database; edit a node, verify patch applied; attempt cross-org write, verify RLS blocks it. Target 90%+ branch coverage on BLoC and validator.