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_decode.py

@@ -23,7 +23,7 @@ from __future__ import annotations
 
 from pathlib import Path
 
-from kernbench.benches._gqa_decode import gqa_decode_kernel  # noqa: F401  (Phase 2 deliverable)
+from kernbench.benches._gqa_decode_long import gqa_decode_long_kernel  # noqa: F401  (Phase 2 deliverable)
 from kernbench.policy.placement.dp import DPPolicy
 from kernbench.runtime_api.bench_runner import run_bench
 from kernbench.runtime_api.types import resolve_device
@@ -72,7 +72,7 @@ def _run_decode_p1(*, h_q: int, h_kv: int):
                       dtype=DTYPE, dp=dp, name=f"o_h{h_q}_kv{h_kv}")
         ctx.launch(
             f"gqa_decode_p1_h{h_q}_kv{h_kv}",
-            gqa_decode_kernel,
+            gqa_decode_long_kernel,
             q, k, v, o,
             T_Q, S_KV, h_q, h_kv, D_HEAD,
             1, 1,                               # C=1, P=1 (no SP, degenerate)