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.py

@@ -1,6 +1,6 @@
 """Phase 1 spec test for P6a GQA prefill kernel (head-parallel, C=1 baseline).
 
-P6a introduces ``_gqa_prefill.py`` with the head-parallel structure (one
+P6a introduces ``_gqa_prefill_long.py`` with the head-parallel structure (one
 Q head per CUBE, per-CUBE distributed output, no reduce). C=1 is the
 degenerate case — no Ring KV, no IPCQ traffic. Validates kernel
 structure and T_q > 1 attention.
@@ -21,7 +21,7 @@ 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.policy.placement.dp import DPPolicy
 from kernbench.runtime_api.bench_runner import run_bench
 from kernbench.runtime_api.types import resolve_device
@@ -62,7 +62,7 @@ def _run_prefill(*, T_q: int, S_kv: int, C: int = 1):
                       name=f"o_t{T_q}_c{C}")
         ctx.launch(
             f"gqa_prefill_p6a_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,