[Unified Cache Linker][3/N]: Add backend-independent linker core

[hzh0425]

Summary

This is the third PR split from #35687.

• Add the backend-independent external linker interface and tree-side wrapper.
• Integrate direct load/offload lifecycle hooks into UnifiedRadixCache.
• Add focused unit tests for linker state and lock handling.

Previous splits: #37091 and #37098.

Test

• test_unified_cache_linker.py: 11 passed on test-2-sglang.
• Ruff and Python compilation checks passed.

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CI States

Latest PR Test (Base): ⏳ Run #33325476798
Latest PR Test (Extra): 🚫 Run #33325605623
Latest PR Test (AMD ROCm 7.2): ❌ Run #33325476754

[hzh0425]

/rerun-test test/registered/unit/mem_cache/test_unified_cache_linker.py

[github-actions]

Results for /rerun-test test/registered/unit/mem_cache/test_unified_cache_linker.py:

🚀 ubuntu-latest (1 test): ✅ View workflow run

cd test/ && python3 registered/unit/mem_cache/test_unified_cache_linker.py

[hzh0425]

/rerun-group radix_cache/unified_radix_tree

[github-actions]

Results for /rerun-group radix_cache/unified_radix_tree:

🚀 4-gpu-h100 (4 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_hicache_pp_kl.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_cp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mamba.py

🚀 4-gpu-b200 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dcp.py

🚀 8-gpu-h200 (3 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4_pp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_glm52.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mimo.py

🚀 2-gpu-h100 (2 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_full.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_swa.py

🚀 1-gpu-h100 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_hybrid_bitexact.py

[hzh0425]

/rerun-test test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py

[github-actions]

Results for /rerun-test test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py:

🚀 ubuntu-latest (1 test): ✅ View workflow run

cd test/ && python3 registered/unit/mem_cache/test_hiradix_pp_sync_drain.py

[hzh0425]