analytical-viz: PE view keeps CP rank locator visible at TP>=8

The Per-PE layout diagram used to color-code + label CP ranks only
when cp_placement=='pe' (CP packed intra-cube). At TP>=8 the TP dim
fills the cube, cp_placement gets pushed to 'cube', the intra-cube CP
branch is skipped, and the CP rank tag silently disappeared from
cubes, per-PE headers, and the figure title. Users at TP>=8 had no
way to tell which of the CP groups the diagram was drawing.

Fix: when cp_placement=='cube' and CP>1, add
  - cube header suffix:   '[CP rank 0 of {CP}]'
  - per-PE header suffix: '  |  CP=0'
  - figure title:         'CP={CP} (showing 1 of CP groups; others identical)'

Behavior at cp_placement=='pe' is unchanged — the existing per-CP-rank
palette + 'CP={rank}' label still fire. CP=1 adds no locators at all.

test_pe_weight_layout covers four cases: TP=8/CP=4 (cube placement,
locator appears), TP=2/CP=4 forced to pe placement (regression:
per-rank labels still there, cube tag not added), CP=1 (no locators),
TP=16/CP=2 (both spilled cubes carry the CP-rank-0 tag).

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

tests/analytical_visualization/pe_weight_layout.py

@@ -114,6 +114,14 @@ def draw_pe_layout(cfg: FullConfig, ax=None):
     cube_h = 6.0
     cube_gap = 0.6
 
+    # When TP fills the cube (TP>=8), cp_placement is "cube" and CP
+    # ranks span cubes — the intra-cube CP-color branch below is
+    # skipped. Surface which CP rank this figure is showing so users
+    # aren't guessing which of `CP` groups is drawn.
+    _cube_place_tag = ""
+    if cfg.topo.cp_placement == "cube" and cfg.topo.cp > 1:
+        _cube_place_tag = f"  [CP rank 0 of {cfg.topo.cp}]"
+
     for cube_idx in range(n_cubes):
         x0 = cube_idx * (cube_w + cube_gap)
         y0 = 0
@@ -125,7 +133,8 @@ def draw_pe_layout(cfg: FullConfig, ax=None):
         )
         ax.add_patch(rect)
         ax.text(x0 + cube_w / 2, y0 + cube_h + 0.1,
-                f"Cube {cube_idx}  (PEs {cube_idx*8}-{cube_idx*8+7})",
+                f"Cube {cube_idx}  (PEs {cube_idx*8}-{cube_idx*8+7})"
+                + _cube_place_tag,
                 ha="center", va="bottom", fontsize=11, fontweight="bold")
 
         pe_pad_x = 0.15
@@ -180,7 +189,14 @@ def draw_pe_layout(cfg: FullConfig, ax=None):
                     _tp_rank = pe_id_in_group % tp
                     _fc = "#f0f7ff"
                     _ec = "#3a86ff"
-                    _cp_label = ""
+                    # cp_placement=="cube" with CP>1: the whole cube is
+                    # one CP rank (rank 0 by convention — see cube title
+                    # tag above). Add the locator so users know what
+                    # rank this per-PE data belongs to.
+                    if _cp_place == "cube" and _cp_val > 1:
+                        _cp_label = "  |  CP=0"
+                    else:
+                        _cp_label = ""
 
                 pe_rect = patches.Rectangle(
                     (px, py), pe_w, pe_h,
@@ -250,9 +266,12 @@ def draw_pe_layout(cfg: FullConfig, ax=None):
     ax.set_aspect("equal")
     ax.axis("off")
 
+    _title_cp = f", CP={cfg.topo.cp}"
+    if cfg.topo.cp_placement == "cube" and cfg.topo.cp > 1:
+        _title_cp += " (showing 1 of CP groups; others identical)"
     title = (
         f"Per-PE layout of one CP group  "
-        f"(TP={tp}, H_q={cfg.model.h_q}, H_kv={cfg.model.h_kv})"
+        f"(TP={tp}{_title_cp}, H_q={cfg.model.h_q}, H_kv={cfg.model.h_kv})"
         f" | KV mode: {cfg.topo.kv_shard_mode}"
     )
     ax.set_title(title, fontsize=11, fontweight="bold")