ZIP-213: Explain how Zebra validates shielded coinbase outputs like other shielded outputs

[teor2345]

Motivation

We've split most of the work out of ZIP-213, so we just need to explain how we implement the remaining consensus rules.

Closes #608.

Specifications

The consensus rules applied to valueBalance, vShieldedOutput, and bindingSig in non-coinbase transactions MUST also be applied to coinbase transactions.

https://zips.z.cash/zip-0213#specification

Note: this rule originally applied to Sapling, but we assume it also applies to Orchard

Solution

• Explain how Zebra validates shielded coinbase outputs like other shielded outputs
• Rearrange some code to make the implementation clearer
• Add a note about mempool coinbase transactions

The existing block test vectors and tests include shielded coinbase transactions.

Review

@jvff is working on the same file.

This PR is not urgent.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

The Sapling and Orchard note consensus rules will be implemented in #2362.

"Only transparent outputs in coinbase transactions are subject to the existing restrictions on spending coinbase funds" will be implemented in #1970:

[dconnolly]

🌱🌳