feat: read-state bootstrap for co-located ephemeral/Regtest followers

[nuttycom]

Draft for discussion, not a merge request. Posted so the team can see the concrete
Zebra-side changes while evaluating the approach in #10786.

Builds atop #10764

Motivation

A co-located read-state follower (e.g. Zallet's zebra-state backend) opens a node's
state DB as a read-only RocksDB secondary and follows its non-finalized best chain over the
indexer gRPC stream. It cannot follow an ephemeral or Regtest node today: an
ephemeral DB lives at a random temp path the follower can't discover, and a follower has no
way to learn a Regtest node's configured activation heights.

Solution

• getreadstateinfo JSON-RPC method (zebra-only): reports the node's live state DB
  path, indexer gRPC address, DB format version/kind, and network identity (kind + genesis
  hash +, for Regtest, activation heights).
• ReadRequest::StateDbInfo → ReadResponse::StateDbInfo: surfaces the live
  finalized-state DB path/version/kind from the read-state service (correct for ephemeral
  DBs, whose path can't be recomputed from config).
• Explicit-path read-only open (Config::read_only_db_path): open a read-only secondary
  at an explicit on-disk path so a follower can tail an ephemeral primary's randomly-located
  DB.
• (Foundational, already on the branch) FindForkPoint read request/helper for reorg
  tracking; don't flush read-only secondary DBs on shutdown.

Testing

• New unit tests: ReadRequest::StateDbInfo reports the live DB; read-only open honors an
  explicit path; getreadstateinfo reports state + Regtest network info.
• cargo fmt --all -- --check, cargo clippy -p zebra-state -p zebra-rpc -p zebrad --all-targets -- -D warnings, and cargo test -p zebra-state -p zebra-rpc pass locally
  (141 + 2 tests).

Notes for reviewers / known limitations

• Custom (non-default) Testnets are not supported for bootstrap. Only Mainnet, the
  default Testnet, and Regtest report enough to reconstruct the network; a custom Testnet
  would be mis-reconstructed as the default public Testnet. Intentional for this scope.
• Exposure: getreadstateinfo returns a local filesystem path and the indexer address.
  It is gated by the same JSON-RPC cookie auth as every method, is read-only, and only
  reflects config the operator already set. The reported indexer_grpc_addr may be a public
  bind address.

AI disclosure

Implemented with Claude Code assistance (design, code, and tests). The contributor is the
sole responsible author.

Closes #10786.

[oxarbitrage]

Looks good overall — I'm fine with the new RPC method for this. I initially considered a file-based approach since this is co-located, but since Zallet already maintains a JSON-RPC connection for steady-state operations (sendrawtransaction, mempool polling), the RPC endpoint is a natural fit for bootstrap discovery.

A few suggestions to make it mergeable:

1. Avoid the None churn in RpcImpl::new — instead of adding indexer_grpc_addr as a constructor parameter (which touches 30+ test call sites), initialize it as None inside new() and add a setter that only start.rs calls. That eliminates ~10 files from the diff.

2. Rebase onto current main — FindForkPoint already landed via #10764, so those changes should drop out of the diff.

Would you mind cleaning it up? Happy to take another look then, looking to merge.

On the testing side, we should also prepare an integration test for the new RPC method and potentially update how we bootstrap in the integration test suite. Zallet's bootstrap path will likely need changes on its side as well to consume getreadstateinfo.