Add prevout union and comments for database metadata.

Author: evoskuil

include/bitcoin/system/chain/prevout.hpp

@@ -32,16 +32,24 @@ class BC_API prevout final
     /// CONSENSUS: 
     /// A height of zero is immature (unspendable) despite unspent state.
     ///************************************************************************
-    /// The confirmed chain height of the prevout (zero if not found).
-    /// Unused if the input owning this prevout is null (coinbase).
-    size_t height{ zero };
+
+    union
+    {
+        /// The confirmed chain height of the prevout (zero if not found).
+        /// Unused if the input owning this prevout is null (coinbase).
+        size_t height;
+
+        /// database: populated with a database identifier for the parent tx.
+        uint64_t parent{ zero };
+    };
 
     ///************************************************************************
     /// CONSENSUS: 
     /// A mtp of max_uint32 fails locktime maturity (until time overflow).
     ///************************************************************************
     /// The median time past at height (max_uint32 if not found/confirmed).
     /// Unused if the input owning this prevout is null (coinbase).
+    /// database: unused as validation precedes prevout block association.
     uint32_t median_time_past{ max_uint32 };
 
     ///************************************************************************
@@ -53,9 +61,11 @@ class BC_API prevout final
     /// all outputs of any duplicate txs are fully spent at height.
     /// If the input owning this prevout is not null (not coinbase), this
     /// indicates whether the prevout is spent at height (double spend).
+    /// database: unused as validation precedes potential spend block assocs.
     bool spent{ true };
 
     /// The previous output is of a coinbase transaction.
+    /// database: populated as does not require prevout block association.
     bool coinbase{ false };
 };