[BugFix][v0.18.0] Gate recompute/balance/fused_mc2 by PD mode (#8374)

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### What this PR does / why we need it?
- Enforce recompute scheduler only in PD-disaggregated mode.
- Enforce balance scheduling only in PD-mixed mode.
- Enforce fused MC2 only on PD-disaggregated D-side (kv_consumer).
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### Does this PR introduce _any_ user-facing change?
No
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Signed-off-by: wangxiaoteng <wangxiaoteng@huawei.com>

docs/source/user_guide/configuration/additional_config.md

@@ -37,7 +37,7 @@ The following table lists additional configuration options available in vLLM Asc
 | `enable_shared_expert_dp`           | bool | `False` | When the expert is shared in DP, it delivers better performance but consumes more memory. Currently only DeepSeek series models are supported. |
 | `multistream_overlap_shared_expert` | bool | `False` | Whether to enable multi-stream shared expert. This option only takes effect on MoE models with shared experts. |
 | `multistream_overlap_gate`          | bool | `False` | Whether to enable multi-stream overlap gate. This option only takes effect on MoE models with shared experts.  |
-| `recompute_scheduler_enable`        | bool | `False` | Whether to enable recompute scheduler.                                                                    |
+| `recompute_scheduler_enable`        | bool | `False` | Whether to enable the recompute scheduler. **Only valid in PD-disaggregated mode** (`kv_role` is `kv_producer` or `kv_consumer`). **Do not enable in PD-mixed mode** (no `kv_transfer_config`, or `kv_role` is `kv_both`); startup will fail with a clear error. |
 | `enable_cpu_binding`                | bool | `True`  | Whether to enable CPU binding. Only takes effect on ARM CPUs; A3 uses the global-slicing CPU allocation strategy and other device types use the topo-affinity CPU allocation strategy. |
 | `SLO_limits_for_dynamic_batch`      | int  | `-1`    | SLO limits for dynamic batch. This is new scheduler to support dynamic batch feature                            |
 | `enable_npugraph_ex`                | bool | `False` | Whether to enable npugraph_ex graph mode.                                                                 |