feat: add sequencer subcommand with embedded store

[sergerad]

Relates to #1961. Implements Phase 3a.

Adds a miden-node sequencer subcommand that runs the store, block-producer, and RPC components in a single process, eliminating the gRPC hops between them for store reads, block application, and transaction submission.

Some of these changes (enums for remote vs local store/block-producer) will be refactored / removed in Phase 4 (cleanup).

TLDR is:

1. miden-node sequencer command
2. sequencer.proto provides sequencer.NtxBuilderApi for ntx-builder to point to
3. replica.proto providers block and proof streaming service

Changes

Store crate — exposes the types needed for in-process use: State, StoreApi, ProvenTipWriter, ApplyBlockError, and a serve_replica function that runs the proof scheduler alongside a new replica.Api gRPC service (see below).

replica.proto — new internal proto that promotes store.StoreReplica out of store.proto into its own replica.Api service with its own descriptor. This decouples the replica streaming interface from the store's client-facing proto surface.

sequencer.proto — new internal proto that promotes store.NtxBuilder out of store.proto into its own sequencer.NtxBuilderApi service with its own descriptor.

Block-producer crate — adds LocalStoreClient (wraps Arc<State> directly) alongside the existing gRPC-based client, unified under a StoreClient enum. Adds EmbeddedBlockProducer, a parallel struct to BlockProducer that takes Arc<State> instead of a store URL. EmbeddedBlockProducer::start() returns a BlockProducerHandle — a cloneable in-process handle that bypasses gRPC for transaction and batch submission — together with a future that drives the gRPC server, batch builder, and block builder.

RPC crate — adds StoreBackend enum (Remote(StoreRpcClient) | Embedded(Arc<StoreApi>)) so RpcService can dispatch store reads in-process. Adds BlockProducerBackend enum (Remote(BlockProducerClient) | Embedded(BlockProducerHandle)) so transaction submission is routed directly to the in-process mempool without a loopback gRPC round-trip. Adds EmbeddedRpc, a parallel struct to Rpc that takes Arc<StoreApi> and Option<BlockProducerBackend> instead of URLs.

bin/node — adds sequencer bootstrap and sequencer start subcommands. start wires the three embedded components together: serve_replica, EmbeddedBlockProducer::start(), and EmbeddedRpc run as concurrent tasks sharing a single Arc<State>. Three service endpoints are exposed: the client-facing RPC API, the block-producer API, and the replica streaming API.

bin/ntx-builder — replaced --store.url with --sequencer.url with the intention being that only the sequencer services ntx-builder queries that are store related.

What stays the same

The existing store, block-producer, and rpc subcommands are untouched — CI stays green throughout. The sequencer is purely additive.

[sergerad]

@Mirko-von-Leipzig having server/embedded.rs doesn't really make sense. But I think potentially this whole block-producer crate might move into the sequencer crate? Or at least we need to just return to this on phase 4 / cleanup to decide how to properly structure it when we remove the RPC path for block producer.

[sergerad]

Note this, another thing for phase 4 / cleanup.