sweep+plot: add primitive hand-tiled (16×16×16) variant to Case-6 decode composite study

Wires a fourth variant into the Cube-SP × PE-SP long-context decode
composite command-form study to surface the worst-case PE_CPU dispatch
inflation that the coarse composite forms delegate to PE_SCHEDULER
(ADR-0065): per-block DMA of Q/K/V slices, tl.dot per 16³ block,
deferred K-inner sum outside the K loop.

Renamed _tiled.py → _hand_tiled_16x16x16.py; coarse-primitive kernel
retained for the 4-cases bench, paper scripts, and the golden byte-equal
regression guard.

New breakdown bar chart at S_kv=128K shows engine (~460 μs) dominates
all three variants; hand-tiled adds ~68 μs PE_CPU dispatch on top —
composite forms sit essentially on the memory-bound floor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/paper/paper_plot_gqa_decode_long_ctx_composite.py

@@ -45,11 +45,12 @@ _N_RANKS = 64  # C·P for the Case-6 64-way split.
 
 # variant key → (display label, colour, marker)
 _VARIANT_STYLE = {
-    "primitive":          ("primitive (tl.dot, hand-tiled)", "#c0504d", "o"),
-    "composite":          ("composite GEMM",                 "#3b6ea5", "s"),
-    "composite_extended": ("composite + softmax_merge",      "#4f8a4f", "^"),
+    "primitive_tiled":    ("primitive hand-tiled (16×16×16)", "#8b5a2b", "D"),
+    "primitive":          ("primitive (tl.dot, hand-tiled)",  "#c0504d", "o"),
+    "composite":          ("composite GEMM",                  "#3b6ea5", "s"),
+    "composite_extended": ("composite + softmax_merge",       "#4f8a4f", "^"),
 }
-_ORDER = ("primitive", "composite", "composite_extended")
+_ORDER = ("primitive_tiled", "composite", "composite_extended")
 
 
 def _load() -> dict: