Structural slot for the add-tab button glyph. 3-stage cascade:
per-instance directive > CNGX_TABS_CONFIG.templates.addIcon >
CNGX_TABS_GLYPHS.addIcon. The button owns its aria-label; the
template paints only the glyph, so its context is void.
The slot paints only the glyph inside the add-tab button. The button shell owns the accessible name, so the template stays decorative.
Accessible name is library-owned. The add button carries aria-label from i18n.addTab, so a glyph-only template is still announced. The slot context is void - it has no label to set.
Do not encode meaning in the glyph alone. The accessible name carries "add tab"; the glyph is a visual cue on top, and decorative icons should be aria-hidden.
Keep the template decorative. The shell is already a <button>; do not nest interactive elements or add SR-only text. Icon only.