Document consensus rules from Zcash spec 7.1.2: Coinbase transactions

[mpguerra]

• A transaction with one or more transparent inputs from coinbase transactions MUST have no transparent outputs (i.e. tx_out_count MUST be 0). Inputs from coinbase transactions include Founders’ Reward outputs and funding stream outputs.
• The total value in zatoshi of transparent outputs from a coinbase transaction, minus vbalanceSapling, minus vbalanceOrchard, MUST NOT be greater than the value in zatoshi of block subsidy plus the transaction fees paid by transactions in this block.
• A coinbase transaction MUST NOT have any transparent inputs with non-null prevout fields, JoinSplit descriptions, or Spend descriptions.
• [Pre-Heartwood] A coinbase transaction also MUST NOT have any Output descriptions.
• [NU5 onward] In a version 5 coinbase transaction, the enableSpendsOrchard flag MUST be 0.
• A coinbase transaction for a block at block height greater than 0 MUST have a script that, as its first item, encodes the block height height as follows. For height in the range {1..16}, the encoding is a single byte of value 0x50 height. Otherwise, let heightBytes be the signed little-endian representation of height, using the minimum nonzero number of bytes such that the most significant byte is < 0x80. The length of heightBytes MUST be in the range {1..5}. Then the encoding is the length of heightBytes encoded as one byte, followed by heightBytes itself. This matches the encoding used by Bitcoin in the implementation of [BIP-34] (but the description here is to be considered normative).
• A transaction MUST NOT spend a transparent output of a coinbase transaction from a block less than 100 blocks prior to the spend. Note that transparent outputs of coinbase transactions include Founders’ Reward outputs and transparent funding stream outputs.
• A transaction MUST NOT spend an output of the genesis block coinbase transaction. (There is one such zero-valued output, on each of Testnet and Mainnet.)
• [NU5 onward] The nExpiryHeight field of a coinbase transaction MUST be equal to its block height.
• [Heartwood onward] All Sapling and Orchard outputs in coinbase transactions MUST decrypt to a note plaintext, i.e. the procedure in § 4.19.3 ‘Decryption using a Full Viewing Key (Sapling and Orchard)’ on p. 66 does not return ⊥, using a sequence of 32 zero bytes as the outgoing viewing key. (This implies that before Canopy activation, Sapling outputs of a coinbase transaction MUST have note plaintext lead byte equal to 0x01.)
• [Canopy onward] Any Sapling or Orchard output of a coinbase transaction decrypted to a note plaintext according to the preceding rule MUST have note plaintext lead byte equal to 0x02. (This applies even during the “grace period” specified in [ZIP-212].)