feat(task15): phase 1 runtime backend abstraction for Studio dataflow + practical example

[vivekrajsingh04]

Closes #38

Summary

Implements Task 15 (Phase 1) by introducing a runtime backend abstraction for Studio dataflow lifecycle, while preserving existing Dora behavior by default.

Changes

• Added RuntimeBackend::{DoraCli, MofaNative} in controller
• Added DataflowController::new_with_runtime(...)
• Kept DataflowController::new(...) default to DoraCli (no behavior change)
• Added explicit unsupported path errors for not-yet-wired native backend:
  • BridgeError::UnsupportedRuntime(...)
• Re-exported runtime backend type from bridge crate
• Added practical example:
  • mofa-dora-bridge/examples/runtime_backend_demo.rs
  • mofa-dora-bridge/examples/README.md
• Added tests:
  • new_with_runtime_sets_backend
  • mofa_native_start_is_explicitly_unsupported_for_now

Validation

cargo check -p mofa-dora-bridge
cargo check -p mofa-dora-bridge --example runtime_backend_demo
cargo test -p mofa-dora-bridge controller::tests::new_with_runtime_sets_backend -- --nocapture
cargo test -p mofa-dora-bridge controller::tests::mofa_native_start_is_explicitly_unsupported_for_now -- --nocapture
cargo run -p mofa-dora-bridge --example runtime_backend_demo -- apps/mofa-fm/dataflow/voice-chat.yml
MOFA_RUNTIME_BACKEND=mofa-native cargo run -p mofa-dora-bridge --example runtime_backend_demo -- apps/mofa-fm/dataflow/voice-chat.yml --start

Scope

Phase 1 only: abstraction + example + tests.
MofaNative lifecycle wiring will be implemented in follow-up PR(s).

[vivekrajsingh04]

@BH3GEI PR is ready for review.

This PR completes Task 15 Phase 1 with:

• runtime backend abstraction (DoraCli / MofaNative)
• practical example under mofa-dora-bridge/examples/
• tests for backend selection and explicit unsupported native lifecycle path

Please let me know if you prefer Task 15 Phase 2 (MofaNative lifecycle wiring) as a separate follow-up PR.

[vivekrajsingh04]

@BH3GEI Sir, gentle ping on #39 when you have a moment.

This PR is scoped to Task 15 Phase 1 only (abstraction + tests + practical example), and I kept Phase 2 (MofaNative lifecycle wiring) separate as follow-up.

Happy to adjust scope/structure based on your review preference.