PAINTROID-761-3: Map and Transform Legacy Commands to Flutter Models

[Amit-Matth]

This Pull Request is the third step of our 4-stage plan to add backward compatibility for legacy .catrobat files. It acts as the core translation layer, successfully mapping the raw binary models reconstructed in Stage 2 directly into modern Flutter commands and canvas states.

PAINTROID-761

New Features and Enhancements

• Draw Command Transformation (LegacyModelTransformer): Implemented mapping logic for core drawing tools, translating LegacySerializablePath and path actions (Move, Line, Quad, Cubic, etc.) directly into Flutter Path objects.
• Paint Mapping: Mapped legacy Paint attributes (color, stroke width, stroke cap, anti-aliasing) directly to modern Flutter Paint configurations.
• Canvas State Command Transformation: Translated complex canvas coordinate and bounds transformations into their Flutter equivalents. Handled state operations including FlipCommand, RotateCommand, CropCommand, ResizeCommand, and ResetCommand.
• Layer Management Translation: Built conversion logic to flatten and map legacy layer states into modern layer implementations (supporting operations like AddEmptyLayerCommand, SelectLayerCommand, RemoveLayerCommand, and LayerOpacityCommand).
• Comprehensive Mapping Unit Testing: Added a complete suite of unit tests under test/unit/serialization/legacy_model_transformer_test.dart to verify the exact mapping parity across all translated commands. All tests are passing locally.

Backward Compatibility Roadmap

To make this feature easy to review and track, the work is organized into four sequential stages:

1. Stage 1 (Completed - Foundational Binary Reader): The low-level KryoReader to parse bytes into basic Dart datatypes (ints, strings, doubles).
2. Stage 2 (Completed - Registry & Model Parser): Setting up the class registry matching the native app's sequential registration IDs to reconstruct raw objects (like CommandManagerModel and ColorHistory).
3. Stage 3 (This PR - Command Transformation): Converting those legacy Android commands (such as historical lines, shapes, and layer state changes) into their equivalent Flutter drawing commands via LegacyModelTransformer.
4. Stage 4 (UI Performance & Fallbacks): Running the decoding logic on background isolates to keep the UI smooth during large file loads, and adding fallback renderers for legacy tools not yet supported by Flutter.

Refactorings and Bug Fixes

• None (New feature architecture).

Checklist

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