Stabilize CP HiCache page-tail ownership under EAGLE reuse

CP shared KV and HiCache now keep page-aligned physical ownership while preserving valid-token radix semantics. Repeated tiny EAGLE exact hits free duplicate tail pages instead of leaking one allocator page, owner-lane load-back uses page-vector admission/eviction, and single-DP idle schedulers avoid entering an unnecessary MLP-sync collective.

The commit also records the current page-aligned cache contract and adds gated decode-side EAGLE accept diagnostics so future accept-length collapses can be tied to draft KV/state transfer evidence instead of more prefill cache speculation.

Constraint: CP HiCache allocator ownership is page-granular while radix matching remains valid-token based.

Constraint: New diagnostics must be gated and must not alter normal EAGLE, transfer, or cache behavior.

Rejected: Padding short requests to cp_size or 2*cp_size pages | wastes KV capacity and still hides valid-tail lifecycle bugs.

Rejected: Adding more unconditional collectives to prove CP consistency | hot-path collectives previously caused severe performance risk.

Confidence: medium

Scope-risk: broad

Directive: Do not reintroduce silent fallback for CP shared KV/HiCache paths; warning-level fallback or fail-fast is intentional.

Tested: git diff --check

Tested: local py_compile for all modified Python files

Tested: remote g0034 container py_compile for modified Python/test files

Tested: remote g0034 container PYTHONPATH=python python -m pytest -q test/registered/unit/layers/test_nsa_cp_utils.py test/registered/unit/mem_cache/test_cp_shared_kv_runtime.py test/registered/unit/mem_cache/test_cp_hicache_load_back_owner_lanes.py test/registered/unit/managers/test_scheduler_dp_attn_mixin.py => 114 passed, 5 warnings, 2 subtests passed

Not-tested: full ETE traffic rerun after this commit

Not-tested: CUDA/TAI kernel benchmark coverage for all production shapes

test/registered/unit/managers/test_scheduler_dp_attn_mixin.py

@@ -0,0 +1,37 @@
+from unittest import TestCase
+from unittest.mock import patch
+
+from sglang.srt.managers.scheduler_dp_attn_mixin import (
+    MLPSyncBatchInfo,
+    prepare_mlp_sync_batch_raw,
+)
+
+
+class _FakeTPGroup:
+    device_group = object()
+    cpu_group = object()
+    device = "cuda"
+
+
+class TestSchedulerDPAttnMixin(TestCase):
+    def test_single_dp_idle_batch_does_not_enter_mlp_sync_collective(self):
+        def fail_all_gather(self, *args, **kwargs):
+            raise AssertionError("idle single-DP scheduler must not all-gather")
+
+        with patch.object(MLPSyncBatchInfo, "all_gather", fail_all_gather):
+            result = prepare_mlp_sync_batch_raw(
+                local_batch=None,
+                dp_size=1,
+                attn_tp_size=1,
+                attn_cp_size=8,
+                tp_group=_FakeTPGroup(),
+                get_idle_batch=lambda: (_ for _ in ()).throw(
+                    AssertionError("idle single-DP scheduler must not build idle batch")
+                ),
+                disable_cuda_graph=False,
+                require_mlp_tp_gather=True,
+                disable_overlap_schedule=True,
+                offload_tags=set(),
+            )
+
+        self.assertIsNone(result)