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_short_context.py

@@ -76,7 +76,7 @@ def _run_decode_short(*, h_q: int, h_kv: int, kv_per_cube: int,
       O: replicated; each group root writes the full byte-conserving
           (h_q·T_q, D_HEAD) result.
     """
-    from kernbench.benches._gqa_attention_decode_short import gqa_attention_decode_short_kernel  # Phase 2
+    from kernbench.benches.gqa_helpers.short_ctx._gqa_attention_decode_short import gqa_attention_decode_short_kernel  # Phase 2
 
     topo = resolve_topology(str(TOPOLOGY_DEFAULT))
 
@@ -196,7 +196,7 @@ def _run_prefill_short(*, h_kv: int, kv_per_cube: int,
     Layout: same head-stacked K/V scheme as decode short, with Q/O
     replicated and byte-conserving reshape inside the kernel.
     """
-    from kernbench.benches._gqa_attention_prefill_short import gqa_attention_prefill_short_kernel  # Phase 2
+    from kernbench.benches.gqa_helpers.short_ctx._gqa_attention_prefill_short import gqa_attention_prefill_short_kernel  # Phase 2
 
     topo = resolve_topology(str(TOPOLOGY_DEFAULT))