Icons now declared on TabContribution via the icon field — the
hardcoded switch in _BottomRail is a fallback only. All sidebar
and context extensions set their Phosphor icon explicitly.
PanelRegistry supports setTabOrder(slot, List<String>) — tabs
render in the order specified by project.layout.sidebar.order /
context.order in settings.yaml. Falls back to registration order.
Priority field removed from all contributions.
Context tabs are static again — persist with their last content,
selection just loads new data and switches focus. No spawn/despawn.
Tickets sidebar priority set to -200 (leftmost, default open).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>