Prevent incompatible CP shared-KV transfer mapping

Mooncake is the only disaggregation transfer backend in this branch with CP shared-KV owner filtering plus logical-page-position destination selection. NIXL still slices destination pages by the original chunk slice, so allowing CP shared-KV prefill on NIXL can silently pair filtered prefill pages with the wrong decode pages.

This keeps the supported path narrow while preserving the page-aligned transfer contract: non-page-aligned valid tails transfer their physical tail page, but do not get padded to CP-size pages.

Constraint: CP shared-KV transfer remaps prefill logical pages to per-rank physical pages while decode metadata remains request-position based.

Rejected: Let NIXL continue through the generic slice path | it lacks logical-page-position selection and can silently corrupt CP shared-KV transfers.

Confidence: high

Scope-risk: narrow

Directive: Do not enable CP shared-KV on another PD transfer backend until its sender filters owner pages and selects decode pages by logical request-page position.

Tested: Local py_compile for server_args and touched tests.

Tested: Remote g0034 pytest test_cp_shared_kv_transfer_mapping.py test_req_to_token_pool.py TestHiCacheArgs: 22 passed, 8 subtests passed.

Not-tested: End-to-end PD transfer with a live non-page-aligned prompt.

Co-authored-by: OmX <omx@oh-my-codex.dev>

test/registered/unit/server_args/test_server_args.py

@@ -415,6 +415,29 @@ class TestHiCacheArgs(CustomTestCase):
                 hicache_storage_backend="mooncake",
             )
 
+    def test_cp_shared_kv_prefill_rejects_nixl_transfer_backend(self):
+        with (
+            self.assertRaisesRegex(
+                AssertionError,
+                "enable_nsa_prefill_cp_shared_kv.*disaggregation_transfer_backend.*mooncake",
+            ),
+            patch.object(
+                ServerArgs,
+                "get_model_config",
+                side_effect=AssertionError("get_model_config should not be called"),
+            ),
+        ):
+            ServerArgs(
+                model_path="dummy",
+                enable_nsa_prefill_context_parallel=True,
+                enable_nsa_prefill_cp_shared_kv=True,
+                nsa_prefill_cp_mode="in-seq-split",
+                disaggregation_mode="prefill",
+                disaggregation_transfer_backend="nixl",
+                page_size=64,
+                enable_hisparse=False,
+            )
+
     def test_hicache_io_backend_and_mem_layout_compatibility(self):
         cases = [
             {