gqa: reorganize benches into gqa_helpers/ subpackage; drop legacy headline

Splits the GQA helpers into a dedicated subpackage to make room for the
prefill 4-cases study (next commit) and a single umbrella bench
(milestone-1h-gqa, after that).

Layout:
  benches/gqa_helpers/
    long_ctx/    — decode 4-cases kernels + sweep runner
    short_ctx/   — prefill/decode short-context kernels
    shared/      — _gqa_panel_helpers + decode_opt2 (context-agnostic)

The registry audit now skips subpackages so gqa_helpers/ (without a
leading underscore) doesn't get audited for @bench decorators.

Also drops the legacy milestone-gqa-headline bench, its
_gqa_attention_prefill_long kernel, 6 dependent prefill tests, the
paper_gqa_latency.py report harness, and the 3 stale headline-derived
PNGs the §6 wire-up referenced (paper will re-pull from the new
1H_milestone_output/gqa/long_ctx/ once §6 is updated).

The _ccl_cfg and _summarize_op_log helpers used to live in the
headline bench; extracted them to gqa_helpers/shared/_gqa_panel_helpers.

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

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

@@ -1,31 +0,0 @@
-{
-  "version": 1,
-  "panels": [
-    "single_kv_group_prefill_gqa_c8_p8"
-  ],
-  "config": {
-    "T_q_prefill": 4,
-    "T_q_decode": 1,
-    "S_kv_prefill": 16,
-    "h_q_decode": 8,
-    "h_kv_decode": 1,
-    "d_head": 64
-  },
-  "rows": [
-    {
-      "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
-      }
-    }
-  ]
-}