Separate Android and Linux CI builds and add transport policy/capabilities

[mortenboye]

Motivation

• Android and Linux use different host MIDI stacks (Android Media MIDI vs ALSA) and should be validated and evolved independently in CI.
• Provide a clear runtime abstraction for which MIDI transports are supported or enabled so platform wrappers can be isolated and apps can opt transports in/out.
• Prepare the repo for a monorepo migration with per-platform packages and Pigeon-based host/flutter contracts.

Description

• Split the CI into dedicated jobs by adding .github/workflows/ci.yml with build_android and build_linux jobs and installing Linux native build dependencies before flutter build linux.
• Introduce transport abstractions with lib/src/midi_transports.dart and wire them into lib/flutter_midi_command.dart to expose MidiCapabilities, MidiTransportPolicy, configureTransportPolicy(...), and transport-gated operations.
• Add monorepo and codegen scaffolding including melos.yaml, pigeons/midi_api.dart, lib/src/pigeon/README.md, doc/architecture.md, and README updates describing the Android/Linux package split.
• Add test/midi_transport_policy_test.dart covering MidiTransportPolicy and MidiCapabilities, and update pubspec.yaml to include melos and pigeon dev dependencies.

Testing

• Ran git diff --check and reviewed the changed files and workflow syntax, which produced no issues.
• Added unit tests in test/midi_transport_policy_test.dart to validate policy resolution and capability helpers, but they were not executed locally in this environment.
• Local flutter/dart commands (flutter analyze / flutter test) were not run because the toolchain is not installed in the execution container, so CI will validate those steps on GitHub Actions.