fix(bench): fill the 3D scene, center the ×, one-line input with send arrow

- The voxel root section wasn't stretched to the scene, so its stage collapsed
to auto height and setupVoxel fell back to a 300px canvas — leaving an empty
blue band below the model. Stretch .buddy-voxel-root (and force the canvas to
fill) so the 3D fills the full 380px scene (verified canvas 380 == scene 380,
gap 0).
- Re-center the × token on the now-full scene (xVsScene ~0).
- Tweak controls on ONE line: widen the row to the scene width + nowrap so the
input and send button no longer stack. Replace the 'Apply' button with a
compact ChatGPT-style accent square showing an up-arrow (↑).

Verified on the fake runtime; 76 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/buddy_fusion/fusion_ui.py

@@ -133,7 +133,9 @@ CSS = (
     ".buddy-slot-clear{position:absolute;top:4px;right:6px;border:none;background:#0001;border-radius:50%;"
     "width:22px;height:22px;cursor:pointer;color:var(--lab-ink);font-weight:700;line-height:1;}"
     # inline per-slot AI tweak inputs (sit under the A / B staged previews)
-    ".buddy-tweak-row{max-width:320px;margin:14px auto 4px;gap:8px;justify-content:center;}"
+    # One line under the scene: input fills, send button sits at the right. Width
+    # matches the scene so input + send never wrap.
+    ".buddy-tweak-row{max-width:440px;margin:14px auto 4px;gap:8px;align-items:center;flex-wrap:nowrap!important;}"
     ".buddy-tweak-row textarea,.buddy-tweak-row input{border-radius:10px!important;text-align:center;"
     "border:1px solid #2a274022!important;background:#ffffffcc!important;color:var(--lab-ink)!important;}"
     ".buddy-tweak-row textarea::placeholder{color:#2a274077;}"
@@ -212,7 +214,7 @@ CSS = (
     # Each side sizes to its scene; flex-wrap lets A/B stack on very narrow screens.
     ".buddy-bench-col{flex:0 1 460px!important;min-width:300px!important;}"
     # × is a soft game-token circle aligned to the scene's vertical centre.
-    ".buddy-bench-x{flex:0 0 auto;align-self:flex-start;margin-top:210px;width:48px;height:48px;border-radius:50%;"
+    ".buddy-bench-x{flex:0 0 auto;align-self:flex-start;margin-top:204px;width:48px;height:48px;border-radius:50%;"
     "display:flex;align-items:center;justify-content:center;font-size:1.6rem;font-weight:800;color:var(--lab-accent);"
     "background:#fff;box-shadow:0 6px 18px #5a93c733,inset 0 0 0 1.5px #ff7a1840;}"
     ".buddy-bench-cell{position:relative;display:flex;flex-direction:column;align-items:center;text-align:center;}"
@@ -225,7 +227,12 @@ CSS = (
     # The big 3D scene: the creature fills it; hint bubbles float over it.
     ".buddy-bench-scene{position:relative;width:440px;max-width:100%;height:380px;border-radius:24px;overflow:hidden;"
     "background:#cfeaff;box-shadow:inset 0 0 0 2px #ffffffcc,0 16px 40px #5a93c733;}"
+    # The voxel root section must FILL the scene, or its child stage collapses to
+    # auto height (parent has no definite height) and setupVoxel falls back to a
+    # 300px canvas — leaving an empty blue band below the model.
+    ".buddy-bench-scene .buddy-voxel-root{display:block;width:100%;height:100%;}"
     ".buddy-bench-scene .buddy-voxel-stage{width:100%;height:100%;border-radius:24px;overflow:hidden;}"
+    ".buddy-bench-scene .buddy-voxel-stage canvas{width:100%!important;height:100%!important;display:block;}"
     # Floating hint bubble: glassy pill positioned over the canvas, gentle bob.
     ".buddy-bubble{position:absolute;z-index:3;width:150px;border:1.5px solid #ffffffcc;"
     "background:rgba(255,255,255,.88);backdrop-filter:blur(4px);border-radius:16px;padding:7px 11px;"
@@ -237,10 +244,13 @@ CSS = (
     "@keyframes buddy-float{0%{transform:translateY(0)}50%{transform:translateY(-5px)}100%{transform:translateY(0)}}"
     "@media (prefers-reduced-motion:no-preference){.buddy-bubble{animation:buddy-float 3.4s ease-in-out infinite;}"
     ".buddy-bubble:nth-child(odd){animation-delay:.9s;}}"
-    # Readable secondary action: soft clay fill + warm-orange ink (was the dark
-    # Gradio default with unreadable dim text). Subordinate to the Splice primary.
-    "#buddy-tweak-a-btn,#buddy-tweak-b-btn{max-width:96px;border-radius:10px!important;font-weight:700;"
-    "background:#fff3e8!important;color:#c2410c!important;border:1.5px solid #ff7a1866!important;}"
+    # ChatGPT-style send: a compact accent square with an up-arrow glyph, sitting
+    # at the right end of the one-line tweak row.
+    "#buddy-tweak-a-btn,#buddy-tweak-b-btn{flex:0 0 auto!important;min-width:46px!important;max-width:46px!important;"
+    "width:46px;height:46px;padding:0!important;border-radius:13px!important;font-size:1.4rem;font-weight:900;line-height:1;"
+    "color:#fff!important;background:linear-gradient(135deg,#ff7a18,#ff4d8d)!important;border:none!important;"
+    "box-shadow:0 4px 14px #ff4d8d44!important;}"
+    "#buddy-tweak-a-btn:hover,#buddy-tweak-b-btn:hover{filter:brightness(1.05);box-shadow:0 6px 18px #ff4d8d55!important;}"
     # ---- Lab entry actions (Compendium + Draw, side by side, centered) -------
     ".buddy-lab-actions{display:flex;gap:10px;justify-content:center;align-items:center;margin:2px auto 4px;}"
     # White secondary pill (matches #buddy-free-enter), subordinate to the tree.
@@ -568,14 +578,14 @@ def build_app() -> gr.Blocks:
                     with gr.Row(elem_classes=["buddy-tweak-row"]):
                         tweak_a = gr.Textbox(show_label=False, container=False, scale=1, lines=1, max_lines=1,
                                              elem_id="buddy-tweak-a", placeholder='✎ or type your own — "add two horns"')
-                        tweak_a_btn = gr.Button("Apply", elem_id="buddy-tweak-a-btn", scale=0)
+                        tweak_a_btn = gr.Button("↑", elem_id="buddy-tweak-a-btn", scale=0)
                 gr.HTML('<div class="buddy-bench-x">×</div>')
                 with gr.Column(scale=1, elem_classes=["buddy-bench-col"]):
                     bench_b_view = gr.HTML()
                     with gr.Row(elem_classes=["buddy-tweak-row"]):
                         tweak_b = gr.Textbox(show_label=False, container=False, scale=1, lines=1, max_lines=1,
                                              elem_id="buddy-tweak-b", placeholder='✎ or type your own — "make it a frog"')
-                        tweak_b_btn = gr.Button("Apply", elem_id="buddy-tweak-b-btn", scale=0)
+                        tweak_b_btn = gr.Button("↑", elem_id="buddy-tweak-b-btn", scale=0)
             splice_btn = gr.Button("Splice! 🧬", elem_id="buddy-fuse-btn", variant="primary")
             # Hidden chip bridge: JS writes {side,prompt} into chip_in then clicks chip_btn.
             chip_in = gr.Textbox(elem_id="buddy-chip-in", elem_classes=["buddy-bridge-box"],