Replace MainActivity's three independent booleans (browsingCode/
browsingHistory/browsingWorkItems) with one typed List<Route> back
stack, restorable through configuration change and process death.
Route holds only stable primitives (owner/name/ref/ref-kind), never
tokens or content. A genuine LIFO stack, not just a refactor: back now
correctly unwinds through arbitrarily nested screens instead of
relying on which boolean happened to still be true underneath another.
No task file existed for this task beforehand (only proposed in an
earlier handoff document); created it first per AGENTS.md.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
