gqa(decode-4cases): give Case 4 its own thin kernel file for naming symmetry

Cases 1-3 each have a dedicated _gqa_attention_decode_<case>.py
kernel file; Case 4 previously reached into _gqa_attention_decode_long.py
via the headline bench's _run_decode_panel helper, breaking the
one-file-per-case convention. Adds _gqa_attention_decode_cube_sp_pe_sp.py
as a 20-line wrapper that bakes in sub_w=4 (the C=8 lrab geometry)
and gives Case 4 its own kind ("decode_cube_sp_pe_sp") and helper
(_run_decode_panel_cube_sp_pe_sp). decode_long.py is unchanged
(still serves the legacy decode_long tests). 16 tests pass.

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

tests/attention/test_milestone_gqa_decode_4cases.py

@@ -98,10 +98,13 @@ def test_case4_panel_registered():
     Headline config:
       C = 8                 (head-parallel CUBE Group)
       P = 8                 (intra-CUBE PE-SP)
-      sub_w = 4             (lrab-adapted center-root reduce; root cube 6)
       T_q = 1               (decode: one new token per pass)
       S_kv = 131_072        (LLaMA long-context decode target)
       d_head = 128, h_q = 8, h_kv = 1
+
+    The lrab sub_w=4 / sub_h=2 geometry is baked into the Case 4
+    wrapper kernel (``_gqa_attention_decode_cube_sp_pe_sp.py``); it
+    is not a panel parameter.
     """
     from kernbench.benches.milestone_gqa_decode_4cases import (
         _PANEL_DISPATCH,
@@ -112,10 +115,11 @@ def test_case4_panel_registered():
     )
     assert _CASE4_PANEL in _PANEL_DISPATCH
     kind, params = _PANEL_DISPATCH[_CASE4_PANEL]
-    assert kind == "decode", f"kind={kind!r}, expected 'decode'"
+    assert kind == "decode_cube_sp_pe_sp", (
+        f"kind={kind!r}, expected 'decode_cube_sp_pe_sp'"
+    )
     assert params.get("C") == 8
     assert params.get("P") == 8
-    assert params.get("sub_w") == 4
     assert params.get("T_q") == 1
     assert params.get("S_kv") == 131_072
     assert params.get("d_head") == 128