Articles: key by label across subtypes, scope per property, FO toggle widget

A single article label (e.g. 'f') may emit on multiple `articles.<subtype>`
lines — `defaults` today, `overwrites` planned — with disjoint property
sets that apply at different stages. Model now merges those emissions:

  { name: 'f', properties: [{ name, value, scope: 'defaults'|'overwrites' }] }

Per-label entries in the AR pane show a property-count breakdown
(`f (1 defaults, 2 overwrites)`). The Article FO pane lists each property
with a `(O-) default` / `(-O) overwrite` toggle — click flips the scope
and marks the score dirty. ASCII glyph mimics a physical switch position
so the active scope is visible at a glance.

Fixture tests cover both the existing single-subtype shape and a synthetic
two-subtype merge (177 assertions total).

static/ast-parser.js

@@ -172,7 +172,7 @@ export function buildModel(rawTree) {
                 break;
             default:
                 if (node.parentSlot === 'articles') {
-                    score.articles.push(buildArticle(node));
+                    mergeArticleEntry(score, node);
                 } else {
                     score[node.slot] = buildGeneric(node);
                 }
@@ -182,6 +182,24 @@ export function buildModel(rawTree) {
     return score;
 }
 
+// Articles are keyed by label. The AST emits one line per (label, subtype)
+// — e.g. `articles.defaults 'f'` and (future) `articles.overwrites 'f'`.
+// We merge them into one entry per label whose properties carry the scope
+// of their originating subtype, so the UI can toggle scope per property.
+function mergeArticleEntry(score, node) {
+    const name = node.positionals[0];
+    let entry = score.articles.find(a => a.name === name);
+    if (!entry) {
+        entry = { type: 'article', name, properties: [] };
+        score.articles.push(entry);
+    }
+    const scope = node.slot; // 'defaults' | 'overwrites' | future subtype
+    for (const [key, value] of Object.entries(node.props)) {
+        entry.properties.push({ name: key, value, scope });
+    }
+    return entry;
+}
+
 function buildTuning(node) {
     const t = { type: 'tuning', base: node.props.base, scales: {}, chords: {}, frequencyFactors: null };
     for (const child of node.children) {
@@ -199,15 +217,6 @@ function buildTuning(node) {
     return t;
 }
 
-function buildArticle(node) {
-    return {
-        type: 'article',
-        subtype: node.slot,
-        name: node.positionals[0],
-        props: { ...node.props },
-    };
-}
-
 function buildInstrument(node) {
     const instr = {
         type: 'instrument',