style(design): FINDING-012 — the create surface is a shell plate

The New Design dialog was the last all-legacy screen: --cad-* surfaces and borders,
an orange focus glow around the brief, English labels (Design Brief, Model Engine,
Tags, Modifiers, Dimensions/Material/Process/Tolerance, Cancel, Generate), three
sets of boxed rows and a 0/5000 counter.

Re-tokenised to the shell: module plate (solid surface, 9px radius, hairline rules),
engraved 零件描述 / 模型引擎 / 标签(可选) / 尺寸·材料·工艺·公差, the density control's
segmented language for 修饰/模板/最近 with the signal-filled active segment, quiet
pill buttons, the accent's soft form for 设置 and AI 润色, shell-field focus on the
brief so the block's own border carries focus instead of a ring inside it, and one
solid signal group: 生成, disabled until there is a brief. Copy is Chinese, matching
the shell (试试例子, AI 润色, 取消, 生成).

Measured: 0 legacy --cad-* computed colours remain inside the dialog in either theme,
0 console errors. DESIGN.md's not-done row now says the surface is re-tokenised while
section 8's single flow (folding templates and case memory into composer recall) is
still not built.