Flesh out consensus accounts ZIP specification

This commit resolves design decisions and adds detailed specifications:

- Decide ZEC-only for initial implementation, with format reserving
  space for future ZSA support via assetClass/assetId fields
- Retain shielded coinbase outputs alongside consensus accounts
- Define consensus_account_id as BLAKE2b-256 hash of protocol type
  and initial public key, stable across key rotations
- Specify account registration bundle (no value, requires signature)
- Specify consensus account output bundle (credits value, no auth)
- Specify consensus account spend bundle with shielding requirement
- Add consensus rule: coinbase registrations require corresponding outputs
- Add ZIP 270 key usage typecode for consensus account spending
- Update ZIP 207/214 changes for funding stream migration
- Add Protocol Specification changes for global state and consensus rules
- Add Rationale and Alternatives sections
- Fix inconsistencies and add missing references

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nuttycom