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/components/PaneFO.js

@@ -153,6 +153,29 @@ export const PaneFO = {
                         ? h(EnvelopeEditor, { basicProperties: node.basicProperties, onChange })
                         : null,
                 );
+            } else if (node.type === 'article') {
+                const markDirty = () => props.store.markDirty();
+                const SCOPES = ['defaults', 'overwrites'];
+                const scopeLabel = s => s === 'defaults' ? '(O-) default' : '(-O) overwrite';
+                children.push(
+                    h('h4', H4, `Article: ${node.name}`),
+                    h('ul', { class: 'se-article-props', style: 'list-style:none;padding:0;margin:0' },
+                        node.properties.map((p, idx) =>
+                            h('li', { key: idx, style: 'display:flex;gap:0.5rem;align-items:center;padding:0.25rem 0' }, [
+                                h('span', { style: 'flex:1' }, `${p.name} = ${p.value}`),
+                                h('button', {
+                                    class: ['se-scope-toggle', `scope-${p.scope}`],
+                                    style: 'font-family:monospace;min-width:8em',
+                                    onClick: () => {
+                                        const i = SCOPES.indexOf(p.scope);
+                                        p.scope = SCOPES[(i + 1) % SCOPES.length];
+                                        markDirty();
+                                    },
+                                }, scopeLabel(p.scope)),
+                            ])
+                        )
+                    ),
+                );
             } else if (node.type === 'bar') {
                 const markBarDirty = () => { node.isDirty = true; props.store.markDirty(); };
                 children.push(