architecture-builderlisted

# Architecture builder Produce `docs/20-architecture.md` and `docs/21-engineering-principles.md` from `docs/00-vision.md` + `docs/10-prd.md`. ## docs/20-architecture.md sections 1. **Platform decision** — iOS / Android / both, with order if sequential. 2. **iOS stack** — - Swift version (latest stable) - Minimum iOS target — resolve `knowledge/stack-defaults.md`'s relative rule (flagship = latest major minus one) to a **concrete number** here, and record how you resolved it - UI: SwiftUI by default; UIKit only if PRD demands something SwiftUI struggles with - State: `@Observable` + `@MainActor` on ViewModels only (Combine / `@Published` / `ObservableObject` are forbidden — `knowledge/ios-conventions.md`) - Networking: URLSession + async/await, or a named lib - Persistence: SwiftData (content apps) or GRDB+SQLite/FTS5 (camera/media) — pick by PRD complexity - DI: lightweight — protocol + initializer injection - Testing: **Swift Testing** (`@Test`/`@Suite`) for unit and domain tests, not XCTest. **UI automation stays XCUITest** — it is XCTest-based, has no Swift Testing equivalent, and `runtime-gate` runs it, so a blanket "never XCTest" would ban the suite the release gate expects. Name the snapshot lib. State the coverage floor for the pure-Swift domain engine package (`knowledge/stack-defaults.md`) 3. **Android stack** — - Kotlin version - minSdk / targetSdk (set based on PRD users) - UI: Jetpack Compose - A