[Backend] Implement gRPC Protocol Adapter for High-Throughput Event Streaming

[DevGod-001]

Summary

Adds a gRPC service using tonic that streams Soroban contract events over HTTP/2 with Protobuf framing, eliminating the latency and serialization overhead of HTTP JSON polling for enterprise indexing consumers.

Changes

• core/proto/events.proto — Defines ContractEvent, StreamContractEventsRequest, and EventStreamService in package soroscope.events.v1
• core/build.rs — Compiles the proto schema at build time via tonic-build
• core/src/grpc.rs — Implements EventStreamServiceImpl backed by the existing SimulationBus broadcast channel; includes unit tests for translate_event filtering
• core/Cargo.toml — Adds tonic, prost, tokio-stream dependencies and tonic-build build-dependency
• core/src/main.rs — Spawns the gRPC server on a configurable GRPC_PORT (default 50051) alongside the existing Axum HTTP server

Usage

GRPC_PORT=50051 RUST_LOG=info cargo run -p soroscope-core

grpcurl -plaintext -d '{"contract_id":"CDLZFC3SYJYDZT7K67VZ75HPJVIEUVNIXF47ZG2FB2RMQQVU2HHGCYSC"}' \
  localhost:50051 soroscope.events.v1.EventStreamService/StreamContractEvents

Validation

• rustfmt --edition 2021 src/grpc.rs — passes
• Unit tests in grpc.rs cover: no-filter passthrough, contract ID match/no-match, event-type filter match/no-match, and progress-event suppression
• CI workspace cargo check --locked --all-targets checks contracts workspace (pre-existing scope; core is standalone as per repo convention)

Closes #583

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