feat: standalone mini event loop for driver-only mode

[FumingPower3925]

Summary

Create a standalone mini event loop that database/cache drivers use when no celeris HTTP server is running. This enables drivers to work independently while still using efficient I/O.

Design

Package: driver/internal/eventloop/

driver/internal/eventloop/
  loop_linux.go    # Standalone mini epoll/iouring loop
  loop_other.go    # Fallback: goroutine-per-conn via net.Conn
  registry.go      # Engine sharing registry

loop_linux.go

• Stripped-down epoll loop (no accept, no HTTP detection, no protocol parsing)
• Manages only DB/cache socket FDs
• Implements the same WorkerLoop interface as the full engines
• Single-worker by default, configurable via WithWorkers(n)
• Clean shutdown: drains pending operations, closes all FDs

loop_other.go (non-Linux fallback)

• Goroutine-per-connection using net.Conn
• Wraps net.Conn read loop to match WorkerLoop callback semantics
• Acceptable performance loss on non-Linux (development/testing)

registry.go

• Global registry where a celeris Server registers its engine as an EventLoopProvider
• Drivers check registry first — if a provider exists, use integrated mode; otherwise spawn standalone loop
• Thread-safe with sync.RWMutex

Acceptance Criteria

• loop_linux.go: functional epoll mini-loop with WorkerLoop interface
• loop_other.go: goroutine-per-conn fallback compiles on macOS/Windows
• registry.go: engine sharing registry with register/lookup/deregister
• Unit tests for both paths
• No dependency on the full engine packages (only engine/ interfaces)