Add P/D connector support matrix and smoke summaries

[Lanoxia]

Summary

• Add docs/PD_DISAGGREGATION.md to track vLLM prefill/decode connector status for kvcached.
• Document the current NIXL smoke-test entrypoint, pass criteria, and known failure signatures from prior P/D investigation.
• Make tools/run_vllm_nixl_pd_smoke.sh write one JSON summary line per completed case to ${LOG_DIR}/summary.jsonl.
• Link the P/D support matrix from the README feature list.

Motivation

Issue #302 asks whether kvcached supports prefill/decode disaggregation, while #311 captured concrete connector-level failures and open validation gaps. This PR turns that context into a repository-level support matrix and a reusable reporting path so future GPU runs can be posted back to issues or PRs with consistent evidence.

Validation

• bash -n tools/run_vllm_nixl_pd_smoke.sh
• git diff --check

Full NIXL validation requires a CUDA/vLLM host with the relevant P/D connector dependencies, so I did not run the end-to-end smoke test on my local macOS environment.

Refs #302

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.