L7: capture-order slot model, card metadata overlay, tertiary-border selection

Add gap-preserving per-bank SlotMap (reorder + Alt-replace mutators, JSON
round-trip, insertion-order migration) to bank_book; stamp captureTimeSig on
Sample; pure card_meta formatters + card_drag gesture/slot module; card
metadata overlay + purple selection border in the panel. Shell drop/cursor
wiring deferred. Fixes: removeSample syncs SlotMap; card_drag gap-probe coordinate.

src/bank_model.cpp

@@ -41,7 +41,9 @@ bool Sample::operator==(const Sample& o) const {
            trackGuids == o.trackGuids && wetDry == o.wetDry &&
            channelCount == o.channelCount && sampleRate == o.sampleRate &&
            lengthSeconds == o.lengthSeconds && lengthBeats == o.lengthBeats &&
-           captureTempo == o.captureTempo && key == o.key && levels == o.levels &&
+           captureTempo == o.captureTempo &&
+           captureTimeSigNum == o.captureTimeSigNum &&
+           captureTimeSigDenom == o.captureTimeSigDenom && key == o.key && levels == o.levels &&
            clipped == o.clipped && tier == o.tier && contentHash == o.contentHash &&
            provenance == o.provenance && createdTimestamp == o.createdTimestamp;
 }
@@ -244,6 +246,8 @@ void writeSample(std::string& out, const Sample& s) {
     w.keyRaw("lengthSeconds", numToStr(s.lengthSeconds));
     w.keyRaw("lengthBeats", numToStr(s.lengthBeats));
     w.keyRaw("captureTempo", numToStr(s.captureTempo));
+    w.keyRaw("captureTimeSigNum", numToStr(s.captureTimeSigNum));
+    w.keyRaw("captureTimeSigDenom", numToStr(s.captureTimeSigDenom));
 
     // Optionals are emitted as null when absent so present/absent round-trips.
     w.keyBegin("key");
@@ -590,6 +594,10 @@ bool Parser::parseSample(Sample& s) {
             if (!parseDouble(s.lengthBeats)) return false;
         } else if (key == "captureTempo") {
             if (!parseDouble(s.captureTempo)) return false;
+        } else if (key == "captureTimeSigNum") {
+            if (!parseInt(s.captureTimeSigNum)) return false;
+        } else if (key == "captureTimeSigDenom") {
+            if (!parseInt(s.captureTimeSigDenom)) return false;
         } else if (key == "key") {
             bool wasNull = false;
             if (!expectNullOr(wasNull)) return false;