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).
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