fix(zebra-consensus): handle overflow panic, double-counting, and coinbase version check

[judah-caruso]

Motivation

This PR fixes the issues reported in #10519, as well as the issues raised in the ZIP-235 audit by Least Authority.

Additionally, this PR fixes code that didn't compile when both nu7 and tx_v6 flags were enabled.

Solution

Tests

New regression tests were added.

Specifications & References

Follow-up Work

AI Disclosure

• No AI tools were used in this PR
• AI tools were used:
  • Claude for basic correctness checks

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.