feat(persist): persist BankBook under banks key, retire legacy bank_index, route capture to active bank (B2)

src/bank_book.cpp

@@ -668,4 +668,24 @@ std::optional<BankBook> BankBook::deserialize(const std::string& json) {
     return book;
 }
 
+BankBook BankBook::loadFromPersisted(const std::string& banksJson,
+                                     const std::string& legacyJson) {
+    // Precedence 1: the authoritative `banks` blob. A present-but-malformed blob is
+    // an error, not an absence — degrade to an empty book rather than falling through
+    // to a stale legacy key (which would resurrect superseded single-bank state).
+    if (!banksJson.empty()) {
+        auto book = deserialize(banksJson);
+        return book ? std::move(*book) : BankBook{};
+    }
+    // Precedence 2: no `banks` yet, but a legacy `bank_index` — one-way pool migration
+    // (deserialize's parse-time legacy path promotes it into the pool). A malformed
+    // legacy blob likewise degrades to empty.
+    if (!legacyJson.empty()) {
+        auto book = deserialize(legacyJson);
+        return book ? std::move(*book) : BankBook{};
+    }
+    // Precedence 3: a brand-new / never-captured project — a fresh empty book.
+    return BankBook{};
+}
+
 } // namespace reasampler