feat: add signature cache

include/xrpl/core/HashRouter.h

@@ -19,14 +19,16 @@ enum class HashRouterFlags : std::uint16_t {
     HELD = 0x08,     // Held by LedgerMaster after potential processing failure
     TRUSTED = 0x10,  // Comes from a trusted source
 
-    // Private flags (used internally in apply.cpp)
-    // Do not attempt to read, set, or reuse.
+    // Private slots: each is claimed by one subsystem to cache a per-tx result.
+    // Don't read or reuse a slot you don't own; the meaning lives at the
+    // constant's definition (e.g. apply.cpp, EscrowFinish.cpp, Batch.cpp).
     PRIVATE1 = 0x0100,
     PRIVATE2 = 0x0200,
     PRIVATE3 = 0x0400,
     PRIVATE4 = 0x0800,
     PRIVATE5 = 0x1000,
-    PRIVATE6 = 0x2000
+    PRIVATE6 = 0x2000,
+    PRIVATE7 = 0x4000
 };
 
 constexpr HashRouterFlags