paper(gqa): rename long-ctx artifacts to gqa_long_ctx_6cases_* and add 4 decode 6-case charts

Filename cleanup so every long-ctx GQA artifact has a consistent
"gqa_long_ctx_6cases_*" prefix (or "gqa_decode_long_ctx_6cases_*"
for decode-only charts). Old "4cases" / mixed names retired.

Renames (long_ctx + figures, content unchanged):
  gqa_hbm_budget.png                  -> gqa_long_ctx_6cases_hbm_budget.png
  gqa_4cases_summary.png              -> gqa_long_ctx_6cases_summary.png
  gqa_4cases_memory_comm_analytical   -> gqa_long_ctx_6cases_memory_comm_analytical.png
  gqa_4cases_memory_comm_paired       -> gqa_long_ctx_6cases_memory_comm_paired.png
  gqa_kv_sharding_6cases_diagram      -> gqa_long_ctx_6cases_kv_sharding_diagram.png
  gqa_kv_sharding_6cases_table        -> gqa_long_ctx_6cases_kv_sharding_table.png
  gqa_3cases_measured_comm.json       -> gqa_long_ctx_6cases_measured_comm.json
  gqa_decode_long_ctx_4cases_*.png    -> gqa_decode_long_ctx_6cases_*.png
                                         (figures dir; long_ctx never had old)

New 4-chart 6-case set in long_ctx output dir (regenerated by
paper_plot_gqa_decode_long_ctx_4cases.py, which now reads all 6
sweep_decode.json panels — Cases 1-6 with the same colour scheme
used elsewhere: red = overflow per-PE HBM, grey = neutral, blue
= Pareto-best ★):

  gqa_decode_long_ctx_6cases_latency.png
  gqa_decode_long_ctx_6cases_memory.png
  gqa_decode_long_ctx_6cases_parallelism.png
  gqa_decode_long_ctx_6cases_traffic.png

Generator scripts updated to write the new filenames + handle the
two new d_head-TP variants (Cases 4, 5) in their per-PE memory and
active-PE-count helpers. Figure widths bumped 10 -> 12 in to fit 6
multi-line case labels.

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

scripts/paper/paper_plot_gqa_kv_sharding_diagram.py

@@ -241,7 +241,7 @@ def _make_table_png() -> Path:
         "(LLaMA 70B GQA single KV-head group · S_kv = 1 M, FP16, 80 layers)",
         fontsize=12, y=0.97,
     )
-    out = _OUT_DIR / "gqa_kv_sharding_6cases_table.png"
+    out = _OUT_DIR / "gqa_long_ctx_6cases_kv_sharding_table.png"
     fig.savefig(out, dpi=150, bbox_inches="tight")
     plt.close(fig)
     print(f"wrote {out}")
@@ -309,7 +309,7 @@ def main() -> Path:
         fontsize=12, y=0.99,
     )
 
-    out = _OUT_DIR / "gqa_kv_sharding_6cases_diagram.png"
+    out = _OUT_DIR / "gqa_long_ctx_6cases_kv_sharding_diagram.png"
     fig.savefig(out, dpi=150, bbox_inches="tight")
     plt.close(fig)
     print(f"wrote {out}")