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>

tests/attention/test_gqa_decode_opt2.py

@@ -151,7 +151,7 @@ def test_k_before_v_in_opt2_plan():
 def test_opt2_bench_completes_oplog_mode():
     from pathlib import Path
 
-    from kernbench.benches._gqa_attention_decode_opt2 import (  # noqa: F401
+    from kernbench.benches.gqa_helpers.shared._gqa_attention_decode_opt2 import (  # noqa: F401
         gqa_attention_decode_opt2_kernel,
     )
     from kernbench.policy.placement.dp import DPPolicy