Implement KNE Packet Bridge daemon and CLI subcommand

[kraney]

1. Overview

This PR introduces the KNE Packet Bridge daemon (bridge.go package) and its CLI subcommand (bridge.go). The packet bridge enables bridging Layer 2 raw Ethernet frames across independent topology segments or linking an external environment (e.g., Borg jobs, developer workstations via veth pairs, or remote KNE clusters) into a KNE cluster over a bidirectional gRPC streaming wire.proto service.

2. Key Components & Architecture

• Wire Server (bridge.go:264):
  • Implements the wpb.WireServer gRPC service (Transmit bidirectional stream).
  • Opens raw AF_PACKET sockets in promiscuous mode with unix.PACKET_OUTGOING echo suppression to prevent infinite frame reflection loops.
  • Uses bridge.go:150 to coordinate a single raw socket reader per interface across multiple concurrent gRPC subscribers.
  • Implements non-blocking channel dispatch with dropped-frame telemetry (droppedFrames) to isolate slow subscribers and prevent socket reader starvation.
  • Integrates bridge.go:40 buffer pooling for 64KB read buffers to minimize heap allocation and GC churn on the packet capture hot path.
  • Automatically evicts dead or errored interface demuxers from the server cache on teardown.
• Symmetric Client (client.go:47):
  • Connects to a remote bridge server and forwards Ethernet frames bidirectionally between a local interface and the peer.
  • Features automatic reconnection and configurable retry delays (--retry_interval).
• CLI Integration (bridge.go:35):
  • Added kne bridge subcommand with server and client modes (plus aliases packet-bridge, packet_bridge).
  • Sets --logtostderr=true by default for seamless container and kubectl logs streaming.
  • Uses signal.NotifyContext for clean termination on SIGINT / SIGTERM.
• Build & Deployment Artifacts:
  • Dockerfile: Minimal multi-stage Alpine container packaging the kne bridge daemon.
  • Makefile:47-65: Added bridge-docker, kind-load-bridge, and bridge-release targets.
• Examples & Documentation:
  • paired-bridge.pb.txt: Example topology demonstrating end-to-end Layer 2 ping between two isolated pods via paired bridge server and client nodes.
  • README.md: Architecture diagrams, verification commands (ping, tcpdump), and instructions for bare-host veth bridging.

3. Testing & Verification

• Unit & Race Tests:
  • Bidirectional packet transmission and buffering tests using in-memory bufconn gRPC connections.
  • Subscriber queue saturation and non-blocking demuxer deadlock verification.
  • Server and client cancellation, retry loops, and config validation tests.
  • All tests pass cleanly under go test -race with zero data races.
• Compatibility: Verified against the full KNE unit test suite with 0 regressions.