Keep Phase8 prefetch on the deferred-consume path

Phase8 only gains useful overlap when the next-layer MLA prefix prefetch is allowed to run until the next layer actually consumes the prefetched buffer. The old wait-after-attention switch let runtime configuration collapse the optimization back into current-layer tail latency, so the prefetch path now has one wait policy and the documentation records the implemented behavior.

Constraint: Phase8 should keep the production environment surface minimal while preserving the existing enable and debug-log knobs
Rejected: SGLANG_CP_SHARED_KV_MLA_PREFETCH_WAIT_AFTER_ATTENTION | it reintroduced current-layer synchronous waiting and made profiling behavior depend on a nonessential policy knob
Confidence: medium
Scope-risk: narrow
Directive: Do not add another Phase8 wait policy knob without first proving the added policy improves end-to-end prefill latency under CP shared KV
Tested: Python AST parse for touched Python files
Tested: git diff --check
Not-tested: Full pytest and remote server integration were not run in this commit

test/registered/unit/mem_cache/test_cp_shared_kv_runtime.py

@@ -257,21 +257,16 @@ class TestCpSharedKVRuntimeHelpers(unittest.TestCase):
         from sglang.srt.layers.attention.nsa.cp_shared_kv_runtime import (
             cp_shared_kv_mla_prefetch_log_enabled,
             cp_shared_kv_mla_prefetch_should_log_layer,
-            cp_shared_kv_mla_prefetch_wait_after_attention_enabled,
         )
 
         envs.SGLANG_CP_SHARED_KV_LOG_MLA_PREFETCH.clear()
-        envs.SGLANG_CP_SHARED_KV_MLA_PREFETCH_WAIT_AFTER_ATTENTION.clear()
         self.assertFalse(cp_shared_kv_mla_prefetch_log_enabled())
-        self.assertFalse(cp_shared_kv_mla_prefetch_wait_after_attention_enabled())
         self.assertFalse(cp_shared_kv_mla_prefetch_should_log_layer(1))
         self.assertTrue(cp_shared_kv_mla_prefetch_should_log_layer(2))
         self.assertFalse(cp_shared_kv_mla_prefetch_should_log_layer(3))
 
         with envs.SGLANG_CP_SHARED_KV_LOG_MLA_PREFETCH.override(True):
             self.assertTrue(cp_shared_kv_mla_prefetch_log_enabled())
-        with envs.SGLANG_CP_SHARED_KV_MLA_PREFETCH_WAIT_AFTER_ATTENTION.override(True):
-            self.assertTrue(cp_shared_kv_mla_prefetch_wait_after_attention_enabled())
 
     def test_token_range_materialize_uses_tai_kernel_when_enabled(self):
         from sglang.srt.layers.attention.nsa import cp_shared_kv_runtime as runtime