[codex] Optimize DeepSeek V4 UCM hit path

[wuhuxiao]

Summary

• Add DeepSeek V4 packed/group0 UCM stores so multi-block external hits only read full tail state for the boundary block.
• Add DeepSeek V4 capture validation tooling and developer validation docs.
• Keep timing diagnostics behind UCM_DEEPSEEK_V4_TIMING=1.

Validation

• python3 -m py_compile ucm/integration/vllm/ucm_connector.py scripts/verify_deepseek_v4_ucm_capture.py
• git diff --check origin/develop..HEAD -- ucm/integration/vllm/ucm_connector.py docs/source/index.md docs/source/developer-guide/deepseek_v4_ucm_validation.md scripts/verify_deepseek_v4_ucm_capture.py
• 1k DeepSeek V4 UCM smoke test: second request hit external: 3, 0.53s; group0 load wait <= ~0.0005s; packed load wait <= ~0.0028s.