docs(design): specify download progress treatment

Document and mock the redesigned downloading state for the launcher. The
reference now replaces the action button slot with a dedicated progress
primitive, covers both card and detail-modal layouts, and records the sizing,
number formatting, container-query fallback, and sample-data expectations that
implementation work should follow.

This commit keeps the design package separate from application code so the
next UI/backend changes can be reviewed against a stable reference.

Test Plan:
- git diff --cached --check

Refs: local design reference update

design/design_reference/SoftLAN Launcher.html

@@ -72,11 +72,16 @@ function App() {
                       initialFilter="local" initialSort="az"
                       initialOpenGame={GAMES.find(g => g.id === 'cod4mw')}/>
           </DCArtboard>
+          <DCArtboard id="detail-modal-downloading" label="E · Detail overlay (downloading)" width={1340} height={840}>
+            <Launcher variant="two" tweaks={t} setTweak={setTweak}
+                      initialFilter="local" initialSort="state"
+                      initialOpenGame={GAMES.find(g => g.id === 'avp')}/>
+          </DCArtboard>
         </DCSection>
 
         <DCSection id="settings" title="Settings dialog"
                    subtitle="Same controls as the dev Tweaks panel, surfaced as an in-app preferences dialog. Open via top-bar menu → Settings.">
-          <DCArtboard id="settings-open" label="E · Settings dialog (open)" width={1340} height={840}>
+          <DCArtboard id="settings-open" label="F · Settings dialog (open)" width={1340} height={840}>
             <Launcher variant="single" tweaks={t} setTweak={setTweak}
                       initialFilter="all" initialSort="recent"
                       initialSettingsOpen={true}/>