ci: add top-level workflow orchestrator

[ndonkoHenri]

https://github.com/flet-dev/flet/actions/runs/21566901473

Summary by Sourcery

Introduce a top-level CI workflow that orchestrates reusable build, release, and macOS integration test workflows based on detected file changes.

Build:

• Split the monolithic CI workflow into a lightweight orchestrator that conditionally calls a reusable build-and-release workflow and a macOS integration tests workflow using changed-file detection.
• Extract the existing build, packaging, publishing, and release jobs into a dedicated build-and-release reusable workflow with shared environment and concurrency configuration.
• Convert the macOS integration tests workflow into a callable workflow so it can be triggered from the main CI orchestrator instead of on every push/path change.

[sourcery-ai]

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[FeodorFitsner]

Isn't it easier to specify files filter on each workflow instead?