refactor: make state invariants explicit

[devmobasa]

Summary

Several paths encoded invariants through separate Option checks followed by another lookup, unwrap, or fallback. This refactor makes those guarantees explicit. No user-facing behavior changes are intended.

• consume owned async and runtime state once
• add required toolbar accessors for guaranteed events, values, segments, steps, and glyphs
• centralize paired GTK popover surfaces, overlay defaults, and performance metadata mappings
• simplify session payload limits, toast preemption, and toolbar persistence routing
• add regression coverage for config mappings, overlay defaults, toolbar glyphs, and visibility persistence