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/realtime_record.cpp

@@ -51,6 +51,8 @@ Sample sampleFromRecordedCapture(const RecordedCapture& cap) {
     s.sampleRate    = cap.sampleRate;   // 0 when project rate was unknown
     s.lengthSeconds = cap.endSeconds - cap.startSeconds;
     s.captureTempo  = cap.captureTempo;
+    s.captureTimeSigNum   = cap.captureTimeSigNum;    // L7 F1 meter stamp (0/0 = unstamped)
+    s.captureTimeSigDenom = cap.captureTimeSigDenom;
     s.tier          = Tier::Scratch;    // captures land in scratch by default
     // contentHash set by the caller (capture_realtime.cpp) after the file is
     // finalized and on disk — the hash is over the finished file bytes. Left empty