bench(milestone-gqa-headline): scale single_kv_group prefill panel T_q=S_kv=1K (scratch budget)

The headline T_q=S_kv=32K target overflows the 1 MB per-PE scratch
pool: at T_q_local=4K and tile_s=1024 the scores matrix alone is 8 MB.
The prefill kernel's bootstrap section also leaves K_t/V_t/scores/
exp_scores persistent (outside tl.scratch_scope), inflating baseline.

Scale-down to T_q=S_kv=1K (T_q_local=128, fits comfortably) preserves
the C=8 + P=8 architecture demonstration; the true LLaMA 32K headline
awaits a future increment to add Q-axis tiling and tighten bootstrap
scratch discipline.

Verified end-to-end: kernbench run --bench milestone-gqa-headline now
produces sweep.json with all 5 panels. The new panel shows
ipcq_copy=896 (matches (C-1)·n_tiles·2·C·P = 7·1·2·8·8) and
dma_write=64 (one per PE, head-parallel + intra-CUBE PE-SP).

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

src/kernbench/benches/1H_milestone_output/gqa_headline/sweep.json

@@ -3,6 +3,7 @@
   "panels": [
     "single_user_prefill_gqa",
     "multi_user_prefill_gqa",
+    "single_kv_group_prefill_gqa_c8_p8",
     "single_user_decode_gqa",
     "multi_user_decode_gqa"
   ],
@@ -39,6 +40,21 @@
         "dma_write_count": 4
       }
     },
+    {
+      "panel": "single_kv_group_prefill_gqa_c8_p8",
+      "kind": "prefill",
+      "C": 8,
+      "P": 8,
+      "T_q": 1024,
+      "S_kv": 1024,
+      "d_head": 128,
+      "op_log_summary": {
+        "gemm_count": 1024,
+        "ipcq_copy_count": 896,
+        "dma_read_count": 192,
+        "dma_write_count": 64
+      }
+    },
     {
       "panel": "single_user_decode_gqa",
       "kind": "decode",