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/components/PaneCP.js

@@ -32,7 +32,7 @@ export const PaneCP = {
             try {
                 const text = await fetchAstLog();
                 props.store.scoreModel = buildModel(parseAstLog(text));
-                props.store.exportLog = [];
+                props.store.resetEditState();
             } catch (e) {
                 importError.value = e.message;
             } finally {
@@ -49,9 +49,7 @@ export const PaneCP = {
                 const model = props.store.scoreModel;
                 const { text: patched, log } = patchScore(
                     raw, model.instruments, model.bars, model.info, model.articles ?? [],
-                    { articlesModified: !!model.articlesModified },
                 );
-                model.articlesModified = false;
                 props.store.exportLog = log;
                 await putScoreText(patched);
                 props.store.synthesisStatus = { frozen: false, currently_rendered_notes: 0, notes_in_total: 0 };