analytical-viz: partial batch (B) scaffold — TopologyConfig field + KV / early stages

WIP toward batch-size support. This first commit is behavior-preserving
at the default B=1 (every new multiplication is by max(1, cfg.topo.b) =
1 today) so all existing tests pass. Follow-up commits will:
  - scale the remaining stages (S6/S7/S8/S9/S10 + C1/C2/C3 + FFN + FFN AR)
  - add a batch selectbox to the sidebar
  - forward b through auto_suggest / auto_explore / auto_hardware

Changes so far:
  - TopologyConfig: new b: int = 1 field (batch size).
  - memory_layout.per_pe_kv_cache_bytes: * B (each concurrent request
    keeps its own KV cache slice).
  - stage_rmsnorm / stage_wq / stage_wkv / stage_kv_append /
    _per_hop_qkT_pv: FLOPs and activation memory scaled by B; weight
    bytes stay fixed (weights shared across the batch).

Verified: 24 pytest tests still pass at default B=1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/analytical_visualization/model_config.py

@@ -43,6 +43,7 @@ class TopologyConfig:
     pp: int = 1                 # pipeline stages (layer shard)
     dp: int = 1                 # data-parallel replicas
     ep: int = 1                 # expert-parallel degree (MoE only)
+    b: int = 1                  # batch size (concurrent requests per PP stage)
     s_kv: int = 4096
     mode: str = "decode"        # "decode" or "prefill"
     kv_shard_mode: str = "split"  # "split" or "replicate" (used when TP > H_kv)