feat(dashboard): rotating Try-this card — one fresh substitution per loop pass (task L-74)

A compact TryThis card follows the playhead chord and shows ONE
suggestion at a time, cycling to the next valid substitution each time
the loop completes a pass (playhead wraps to a lower station). Over
Am-C-F the F cycles Dm -> Fm -> Fmaj7 -> E7 across passes, so the app
keeps offering a new idea and eventually teaches every honest move,
never a wrong one. Chip taps into ChordDetailModal; 0 subs -> no card,
1 sub -> static. Mounted App.jsx-only above RelatedProgressions in the
related slot; audio contract grep-clean; rotation logic exported pure
and StrictMode-safe. Critic PASS (2-pass rotation trace verified).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/App.jsx

@@ -11,6 +11,7 @@ import DrumView from './components/DrumView'
 import { NOTES, detectKey, detectTopKeys, matchChordFromChroma, detectRepeatingProgression, getChordTones, getChordCandidates, getNoteHistoryAnalysis } from './lib/theory'
 import ChordDetailModal from './components/ChordDetailModal'
 import RelatedProgressions from './components/RelatedProgressions'
+import TryThis from './components/TryThis'
 import LoopStation from './components/LoopStation'
 import JamGuide, { KnowledgeDock } from './components/JamGuide'
 import { useLoopEngine } from './services/loopEngine'
@@ -854,11 +855,19 @@ export default function App() {
           </>
         }
         relatedSlot={
-          <RelatedProgressions
-            loop={detectedProgression}
-            keyInfo={effectiveKey}
-            onChordClick={setSelectedChord}
-          />
+          <div className="flex flex-col gap-3">
+            <TryThis
+              loop={detectedProgression}
+              keyInfo={effectiveKey}
+              currentChord={currentChord}
+              onChordClick={setSelectedChord}
+            />
+            <RelatedProgressions
+              loop={detectedProgression}
+              keyInfo={effectiveKey}
+              onChordClick={setSelectedChord}
+            />
+          </div>
         }
         fill={jamView}
       />