feat(rpc): expose the Ironwood note commitment tree and subtrees

[upbqdn]

Motivation

The Ironwood state (#10762) stores a note commitment tree and subtrees, but no read request or RPC exposes them, so a wallet or light-client indexer cannot obtain Ironwood frontiers or subtree roots after NU6.3 — the pool is spendable per consensus but unusable through Zebra's RPC surface. This adds the read path, mirroring Orchard. Stacks on #10886.

Solution

• state: ReadRequest::IronwoodTree / ReadRequest::IronwoodSubtrees and the matching ReadResponse variants; read::ironwood_tree / read::ironwood_subtrees; the ZebraDb::ironwood_tree_by_hash_or_height accessor; and the read-service handlers. These mirror the Orchard read path (Ironwood reuses the Orchard tree/node types in separate column families).

• rpc:

  • z_gettreestate gains an ironwood treestate, present only from NU6.3.
  • z_getsubtreesbyindex accepts pool = "ironwood".
  • verbose getblock reports the Ironwood note-commitment-tree size under trees.ironwood.

  All three are Option/skip_serializing_if-empty and serde(default) on read, so pre-NU6.3 responses and existing fixtures are byte-for-byte unchanged.

Tests

Local gate green on d.lan: fmt --check, clippy --workspace --all-targets -D warnings, cargo doc, and nextest for zebra-rpc (107) and zebra-state (149). The existing z_gettreestate / getblock snapshot and serialization round-trip tests pass unchanged (Ironwood is empty/absent pre-NU6.3); the GetBlockTrees round-trip test now also carries the empty Ironwood tree.

Specifications & References

• Follow-up to the review of feat!: Support Ironwood and v6 transactions #10762 / fix!: address Ironwood review findings across mempool, consensus, rpc, and state #10884; the RPC read-path gap split out from fix!: address more Ironwood review findings across chain, consensus, and state #10886.

Follow-up Work

None outstanding.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Claude found the gap in the review, implemented the read path, and drafted this description.

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.

[arya2]

Looks good pending a fix for the getblock RPC returning error for heights below NU6.3

The base branch was changed.

[conradoplg]

Not sure how it does not have conflicts, but should probably be rebased after #10886 merged

[conradoplg]

Admin-merging, tested locally