Add a consensus rule that the (zero-valued) coinbase transaction output of the genesis block cannot be spent.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -9159,6 +9159,8 @@ $\versionField \geq 4$ and $\nShieldedSpend + \nShieldedOutput > 0$.
   \item A \transaction{} \MUSTNOT spend a \transparent output of a \coinbaseTransaction
         from a \block less than 100 \blocks prior to the spend. Note that \transparent outputs of
         \coinbaseTransactions include \foundersReward outputs \canopy{and \transparent \fundingStream outputs}.
+  \item A \transaction{} \MUSTNOT spend an output of the \genesisBlock{} \coinbaseTransaction. (There is one
+        such zero-valued output, on each of \Testnet and \Mainnet.)
   \overwinteronwarditem{\nExpiryHeight{} \MUST be less than or equal to 499999999.}
   \overwinteronwarditem{If a \transaction is not a \coinbaseTransaction and its \nExpiryHeight{} field
         is nonzero, then it \MUSTNOT be mined at a \blockHeight greater than its \nExpiryHeight.}
@@ -10881,6 +10883,8 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
     \item Add a missing consensus rule that has always been implemented in \zcashd: there
           must be at least one \transparentOutput, \outputDescription, or \joinSplitDescription
           in a \transaction.
+    \item Add a consensus rule that the (zero-valued) \coinbaseTransaction output of the
+          \genesisBlock cannot be spent.
     \item Define \SproutChainValuePoolBalance\sapling{ and \SaplingChainValuePoolBalance,} and include
           consensus rules from \cite{ZIP-209}.
 \sapling{