PAINTROID-761: Implement foundational Kryo binary reader in Dart

[Amit-Matth]

This Pull Request is the first step of our 4-stage plan to add backward compatibility for legacy .catrobat files. It introduces the low-level binary reader in Dart to decode raw Kryo streams.

PAINTROID-761

Note: The comments in the code have been kept to make review of the binary format easier, and they will be removed once the PR has been reviewed.

New Features and Enhancements

• Implemented Foundational Kryo Binary Reader (KryoReader): Created a high-performance binary reader under lib/core/backward_compatibility/kryo_reader.dart to decode primitive data types (integers, floats, doubles, booleans, characters).
• Varint and Varlong Decoding: Added support for variable-length integers and longs used in Kryo serialization, including ZigZag decoding for signed compression.
• Kryo String Protocol: Implemented full Kryo string specifications (handling null strings, empty strings, and UTF-8/ASCII bytes) including support for Kryo's custom ASCII-optimization flag.
• Robust Unit Testing: Added comprehensive unit tests under test/unit/serialization/kryo_reader_test.dart to verify navigation, fixed primitives, varints, and optimized strings. All tests are passing locally.
• Isolated Implementation: Kept all new code separated under lib/core/backward_compatibility/, ensuring zero impact on the existing codebase during review.

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Backward Compatibility Roadmap

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

1. Stage 1 (This PR - Foundational Binary Reader): The low-level KryoReader to parse bytes into basic Dart datatypes (ints, strings, doubles).
2. Stage 2 (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 (Command Transformation): Converting those legacy Android commands (such as historical lines, shapes, and layer state changes) into their equivalent Flutter drawing commands.
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 foundation).

Checklist

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