fix(design): FINDING-007 — the eight plates form a heading outline

The workspace rendered zero h1-h6, so a screen reader met 114 tab stops and no
structure. Module titles are now h2 (the brand plate keeps a span, it carries a
wordmark rather than a section name).

Measured after: H2 零件槽位 / 尺寸 / 检验 / 源码 / 读数 / 视口 · 3D / CONTROLS.
Styling is unchanged; the engraved 9px mono label is still what the eye reads.