gqa: rename long-context kernels to *_long for symmetry with *_short

Make the file + function naming symmetric:
  _gqa_decode.py        -> _gqa_decode_long.py
  _gqa_prefill.py       -> _gqa_prefill_long.py
  gqa_decode_kernel     -> gqa_decode_long_kernel
  gqa_prefill_kernel    -> gqa_prefill_long_kernel

Mirrors the existing _gqa_{decode,prefill}_short.py naming. Updates the
two imports + two call sites in milestone_gqa_headline.py and the 9
attention tests that import the kernels.

Tests: 72/72 focused regression green (tests/attention/ + Phase E + TL
discipline). milestone-gqa-headline bench passes its 7 panel/schema
assertions.

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

tests/attention/test_gqa_prefill_ring.py

@@ -34,7 +34,7 @@ from __future__ import annotations
 
 from pathlib import Path
 
-from kernbench.benches._gqa_prefill import gqa_prefill_kernel  # noqa: F401
+from kernbench.benches._gqa_prefill_long import gqa_prefill_long_kernel  # noqa: F401
 from kernbench.ccl.install import load_ccl_config, resolve_algorithm_config
 from kernbench.ccl.sfr_config import configure_sfr_intercube_ring  # noqa: F401 (Phase 2)
 from kernbench.policy.placement.dp import DPPolicy
@@ -89,7 +89,7 @@ def _run_prefill_ring(*, T_q: int, S_kv: int, C: int):
                       name=f"o_t{T_q}_c{C}_ring")
         ctx.launch(
             f"gqa_prefill_ring_t{T_q}_s{S_kv}_c{C}",
-            gqa_prefill_kernel,
+            gqa_prefill_long_kernel,
             q, k, v, o,
             T_q, S_kv, D_HEAD, C,
             _auto_dim_remap=False,