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.h

@@ -86,6 +86,13 @@ struct Sample {
     double lengthBeats   = 0.0;
     double captureTempo  = 0.0;     // project tempo (BPM) at capture time
 
+    // Time signature at capture time (L7 F1 — stamped alongside captureTempo so the
+    // bars.beats.subdivisions read-out is stable under later project meter changes).
+    // 0/0 means UNSTAMPED (pre-L7 sample, or a capture that could not read the meter);
+    // the metadata formatter renders a blank musical read-out for 0/0 and keeps s.ms.
+    int    captureTimeSigNum   = 0;  // meter numerator   (e.g. 4 in 4/4); 0 = unstamped
+    int    captureTimeSigDenom = 0;  // meter denominator (e.g. 4 in 4/4); 0 = unstamped
+
     std::optional<std::string> key;  // musical key, when known
 
     Levels levels;