phases 4-6: article system, stem note fields, edit-state cleanup

Phase 4 — remove edit-state flags from domain objects:
- ast-parser.js: drop isDirty from buildInstrument, buildBar, buildStemNote
- store.js: add resetEditState()
- PaneCP.js: use resetEditState() on import; drop flags arg from patchScore
- PaneFO.js: isDirty -> _modified for linked instruments; no per-node dirty tracking
- PaneSubObjects.js: _spliceNode helper; deletion via splice; _isNew on new bars
- exporter.js: on-demand traversal; absence from model means deleted

Phase 5 — article parser and exporter:
- ast-parser.js: _buildArticleEntry replaces _mergeArticleEntry; stage.article and
  voice.article dispatch; article properties carry scope and overwritten flag
- exporter.js: _buildArticlesBlock emits -important: list for overwrites scope

Phase 6 — stem note dead fields:
- ast-parser.js: parse weight and articulatory on stem notes; flatten
  article.definite properties onto stem note model sibling to pitch/chain

static/store.js

@@ -25,4 +25,10 @@ export const store = reactive({
     markDirty() {
         this.isDirty = true;
     },
+
+    resetEditState() {
+        this.isDirty = false;
+        this.exportLog = [];
+        this.focusPath = [];
+    },
 });