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/persist.cpp

@@ -597,6 +597,14 @@ void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDi
         book_ = BankBook::loadFromPersisted(std::string{}, legacyJson);
     }
 
+    // L7 slot migration: seed every bank's display-position SlotMap from its index
+    // insertion order when the loaded blob carried none (a pre-L7 project -> dense,
+    // gap-free, visually identical on first post-L7 load), and reconcile a partial map
+    // (drop stale markers, append unmapped samples) for a blob written by an earlier L7
+    // build. One-way: once the book is re-saved the reconciled slot data is authoritative.
+    // Idempotent, so a fresh empty book is a cheap no-op.
+    book_.reconcileSlots();
+
     // Project-relative resolution is a READ-time concern: every BankIndex in the book
     // stores only relative paths (invariant, enforced per-bank at add()), and consumers
     // (M5 panel, M6 insert) resolve each entry against the CURRENT project dir via