Storage: add native multi-get support (QMDB + API)

[tac0turtle]

Context

We experimented with adding multi_get at the ReadonlyKV/storage-wrapper level in evolve_storage, but the current backend (commonware_storage::qmdb in use here) exposes single-key get and write batching, not native read batching.

That means wrapper-level multi_get can reduce some overhead, but remains fundamentally N x single-key reads.

Problem

Hot read paths (cache warming, account/state lookups, RPC-adjacent storage reads) need deterministic, ordered batch reads without paying per-key API/runtime overhead repeatedly.

Proposal

1. Add native read-side batch API support in storage backend integration:
   • Target API shape in our layer: multi_get(&[Vec<u8>]) -> Vec<Option<Vec<u8>>> (input-order preserving).
   • If upstream QMDB adds a native batch read, use it directly.
   • If not, implement the best possible pipelined/parallel strategy with bounded concurrency and deterministic output ordering.
2. Keep cache behavior explicit:
   • Resolve cache hits first.
   • Fetch misses in batch path.
   • Backfill positive and negative cache deterministically.
3. Adopt in first consumer path:
   • cache warming (warm_cache)
   • then one RPC/state query hot path
4. Add benchmarks + regression gates:
   • compare get loop vs multi_get for cold/warm cache
   • include key cardinality and hit-ratio variants

Acceptance Criteria

• Deterministic output order for any key vector (including duplicates).
• No behavior regressions vs existing single-key get.
• Measurable latency improvement on representative read-heavy workloads.
• Benchmarks checked into repo and runnable in CI/dev.

Notes

• This should be done as small reversible steps: API -> backend impl -> one consumer -> benchmarks.
• Keep memory bounds explicit for any parallel/pipelined implementation.