Choose an App Architecture

Background: Native [iOS SwiftUI+Combine / Android Jetpack Compose / both]
app. Here's what it does and how big it's likely to get:
[describe the app, feature count, and expected growth]
Team: [solo / size]. Experience level: [junior / mixed / senior].

Use case: Help me choose an app architecture and commit to it, rather
than inventing structure ad hoc as I go.

Implementation details: Compare the realistic options for my platform:
- iOS: MVVM (+ Combine), MVVM-C (coordinators), TCA (Composable
  Architecture), or Clean Architecture
- Android: MVVM (+ StateFlow), MVI, or Clean Architecture
Weigh them against my app size, team experience, testability needs, and
how much boilerplate each imposes. Don't over-engineer a small app.

Layer & dependencies: I want clear boundaries between UI, business logic,
and data so I can test logic without the UI and swap data sources later.

Deliverable: A recommended architecture with reasoning, a comparison
table against my constraints, a diagram (described in text) of how data
and events flow, and the main downside of the recommended choice.