[feature] add bridge utxo indexing and grpc endpoint

[lowhung]

Description

This PR adds partner-chain bridge observability data to midnight-state in Acropolis for the preview network. It indexes UTXOs created at the Illiquid Circulation Supply (ICS) validator address for the configured bridge asset, computes tokens_out and tokens_in with the same checkpoint semantics used by the partner-chains db-sync bridge reader, and exposes that data through a new GetBridgeUtxos gRPC endpoint.

The user-visible effect is that Acropolis can now serve raw bridge ICS UTXOs directly over gRPC without relying on db-sync for this data path. The endpoint is intentionally UTXO-granular rather than transaction-granular so downstream consumers can resume from either a block checkpoint or a specific UTXO checkpoint without duplicates.

Related Issue(s)

None.

How was this tested?

• cargo fmt
• cargo test -p acropolis_module_midnight_state
• cargo clippy -p acropolis_module_midnight_state --all-targets -- -D warnings
• cargo check -p acropolis_process_midnight_indexer

Checklist

• My code builds and passes local tests
• I added/updated tests for my changes, where applicable
• I updated documentation (if applicable)
• branch has ≤ 5 commits (honor system)
• commit messages tell a coherent story
• branch is up to date with main (rebased on main; fast-forward possible)
• CI/CD passes on the merged-with-main result

Impact / Side effects

This PR changes the midnight-state protobuf contract by adding a new bridge RPC and related messages, so downstream gRPC consumers will need to vendor the updated proto before they can call the new endpoint. The indexing change is additive: existing cNIGHT, candidate, governance, and parameter endpoints remain intact, and the branch includes the latest upstream stable-block gRPC additions.

On the indexing side, bridge tokens_in is resolved from previously indexed ICS UTXOs plus same-block temporary bridge outputs so same-block ICS spends can be represented correctly before the block is committed. Missing or non-inline datums are treated as absent rather than failing indexing.

Reviewer notes / Areas to focus