Add the first-party native Taskboard reference app - #103

Summary

• add a maintained Taskboard reference app with one Racket backend and independent WinUI 3, SwiftUI, and GTK4 list/detail interfaces
• exercise Records, Enums, typed RPC, Event, State, resources, cancellation, bounded progress, notifications, empty/loading/error states, and UI-thread dispatch
• add stable accessibility identifiers, bilingual guided walkthroughs, a schema baseline, executable backend behavior/performance tests, and explicit native performance budgets
• build, package, verify, and clean the reference app on Windows, macOS, and Linux CI; prove the example ships in the source package
• make generated starter cleanup cover native build products and document generated-client ownership

Local verification

aco test tests/ — 8,575 tests passed

aco test tests/codegen.rkt tests/documentation.rkt tests/reference-taskboard.rkt — 237 tests passed after the final edits

aco rivet schema check rivet-schema.json --json — compatible

• Windows Debug native build — succeeded after the final UI changes
• Windows Release package + verify — succeeded, including packaged sample resources
• source package archive — contains both walkthroughs, performance contract, backend, and all three native hosts; clean archive installation reached the existing Windows-only python3 limitation after the new installed-file checks passed

This is a substantial implementation step toward #88. It intentionally does not close the issue: screenshot baselines and platform accessibility-tree/interaction automation still need their own evidence before that checklist is complete.