Dict-like metadata interface & private metadata

[yfukai]

Solves #192

This pull request refactors the metadata API for graph backends, introducing a new MetadataView class to provide a dictionary-like interface for metadata that synchronizes mutations directly with the graph.

Metadata API udpate:

• Introduced the MetadataView class in src/tracksdata/graph/_base_graph.py, which provides a dictionary-like interface for metadata and ensures that updates, removals, and other mutations are synchronized with the graph backend. This replaces the previous metadata() and update_metadata() methods.
• Refactored the BaseGraph class and its subclasses to expose metadata via the metadata property (public) and _private_metadata property (private), with enforced validation to prevent accidental modification of private metadata from public APIs. Backend-specific methods for updating and removing metadata are now internal (_update_metadata, _remove_metadata).

Private metadata handling and validation:

• Added strict validation and separation of public and private metadata keys, preventing accidental modification of internal metadata from public APIs and providing a dedicated _private_metadata property for internal use. A new test ensures that private metadata is hidden from public APIs and that validation errors are raised for improper access.

API and import changes:

• Updated src/tracksdata/graph/__init__.py to export MetadataView and reflect the new API, ensuring consistent imports and usage across the package.
• Adjusted logging and documentation strings to reference the new property-based metadata API.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.91011% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.87%. Comparing base (e10d8c3) to head (68b01d4).

Files with missing lines	Patch %	Lines
src/tracksdata/graph/_base_graph.py	69.44%	20 Missing and 2 partials ⚠️
src/tracksdata/graph/_graph_view.py	66.66%	2 Missing ⚠️
src/tracksdata/nodes/_regionprops.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
- Coverage   88.18%   87.87%   -0.32%     
==========================================
  Files          56       56              
  Lines        4308     4378      +70     
  Branches      749      758       +9     
==========================================
+ Hits         3799     3847      +48     
- Misses        319      339      +20     
- Partials      190      192       +2     

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