Refactor: decouple solver runtime & API services

[chris-ricketts]

Closes #2

Summary

• Refactors solverd so runtime orchestration is separate from API/application services.
• Keeps TakerService focused on pair, trade, wallet, and balance operations.
• Keeps PreimageService focused on exposing preimage metadata.
• Runs banco and preimage plugins through one shared solver runtime.
• Extracts reusable runtime wiring into internal/solverd.Run and SetupWallet.
• Lets e2e tests use the same runtime path as production.
• Updates e2e tests to interact through the real gRPC API.
• Adds deterministic price feed injection for tests.
• Makes closed plugin streams explicit with solver.ErrAllStreamsClosed.
• Switches gRPC server setup to option-based service registration.