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/capture.cpp

@@ -53,6 +53,7 @@
 #define REAPERAPI_WANT_Main_OnCommand
 #define REAPERAPI_WANT_Main_SaveProject
 #define REAPERAPI_WANT_Master_GetTempo
+#define REAPERAPI_WANT_TimeMap_GetTimeSigAtTime
 #include "reaper_plugin_functions.h"
 
 namespace reasampler {
@@ -498,6 +499,21 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
     s.sampleRate    = effectiveSampleRate;  // 0 when project rate was unknown
     s.lengthSeconds = request.endSeconds - request.startSeconds;
     s.captureTempo  = Master_GetTempo();   // BPM at capture time (verified ~4651)
+    // Time signature at the capture's START time (L7 F1 stamp). TimeMap_GetTimeSigAtTime
+    // (verified reaper_plugin_functions.h:7130 — void(ReaProject*, double time,
+    // int* numOut, int* denomOut, double* tempoOut)) reads the meter effective at that
+    // project time, so a sample captured under 3/4 keeps a 3/4 read-out even if the
+    // project later switches to 4/4. proj=nullptr => the active project (matches the
+    // Master_GetTempo() call above, which is also active-project). The tempoOut is
+    // ignored — captureTempo already carries the master tempo. Leaves 0/0 (unstamped)
+    // if the API is somehow unavailable; the formatter renders a blank musical read-out.
+    {
+        int tsNum = 0, tsDenom = 0;
+        double tsTempo = 0.0;
+        TimeMap_GetTimeSigAtTime(nullptr, request.startSeconds, &tsNum, &tsDenom, &tsTempo);
+        s.captureTimeSigNum   = tsNum;
+        s.captureTimeSigDenom = tsDenom;
+    }
     s.tier          = Tier::Scratch;       // captures land in scratch by default
     // Content hash: WAV-aware FNV-1a over the rendered file's fmt+data chunks so
     // hashReferencedElsewhere can identify copies in other banks and suppress the