feat(xtcp): stdout destination + envelope JSON/text marshallers

[randomizedcoder]

Summary

Adds a stdout destination so xtcp2 can print socket data to its own stdout — e.g.:

xtcp2 -dest stdout -marshal protoJson -d 1

streams one JSON Envelope per poll cycle as NDJSON. Great for local development, debugging, and piping to jq. In Docker, the daemon's records appear directly in docker logs (logs go to stderr, so stdout carries only the JSON).

What's included

• Reusable destination — backed by an io.Writer-based writerDest (default os.Stdout) instead of copying the null/udp Send/Close boilerplate. A future stderr/file sink is a one-line factory over the same type. The io.Writer seam is also the test seam (inject a bytes.Buffer).
• Latent bug fix — the destination pipeline is envelope-based, but only protobufList was registered as an EnvelopeMarshaller. So -marshal protoJson|protoText|msgpack with any destination nil-deref panicked at flushEnvelope. This registers envelope-level marshallers for all three; protoJson uses protojson.Marshal (compact, one object per line).
• Validation — stdout is treated like null (both bare stdout and stdout: accepted).
• -dest help mentions stdout.

Tests

• writerDest framing (payload + newline, no buffer mutation), both write-error branches, and factory wiring — all via the io.Writer seam, no os.Stdout juggling.
• Envelope marshaller resolution for protoJson/protoText/msgpack, and that the JSON output is valid one-line JSON with a row array.
• Validation happy-paths for stdout / stdout:.

Verification

• nix build .#test-pkg-xtcp, .#test-cmd-xtcp2, .#checks.x86_64-linux.golangci-lint all pass.
• End-to-end in a container: -dest stdout -marshal protoJson -d 1 printed one-line JSON envelopes of real host sockets (105 rows, full tcp_info / congestion / skmem / cgroup fields) to stdout.

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