Implement ZIP-0227 global issued-assets state in Zebra

[dmidem]

This PR adds global issued-asset state tracking for ZSA issuance per ZIP-0227.

Key changes:

• Introduces AssetState / IssuedAssetChanges and integrates Orchard’s ZSA validation APIs to compute per-block issued-asset deltas.
• During consensus checks, it uses Orchard’s validate_bundle_burn and verify_issue_bundle to validate burns and issuances while building the global issued_assets state. This relies on the Orchard refactoring in Refactor issue/burn validation for Zebra asset state integration orchard#199, which exposes/reworks these validation helpers for direct integration from Zebra.
• Threads Orchard transaction sighashes through contextual block/tx verification so verify_issue_bundle can check issuance authorization signatures (issueAuthSig).
• For checkpoint-verified blocks, it uses verify_trusted_issue_bundle instead of verify_issue_bundle because the current Zebra checkpoint-verification path does not provide transaction sighashes. This is intentional: checkpoint-verified blocks are treated as already verified and are only replayed through the global-state logic to (re)build issued_assets during node startup, so signature checking is not required there.
• Persists issued-asset state in Zebra’s finalized DB, applies updates in non-finalized and finalized states with rollback support, and exposes a read path.
• Adds an RPC method to query current asset state by AssetBase, plus tests/snapshots and workflow vectors.

Notes:

• Issued-note rho derivation follows ZIP-0227 by passing nf_{0,0} (the first Orchard Action nullifier) into issuance verification.

[dmidem]

Closed in favor of #111