feat: replace IAssetResolver + IAssetCatalog with IAssetSource

[jonisavo]

Combine the separate catalog (path resolution) and resolver (asset loading) interfaces into a single IAssetSource interface. Convention resolution moves inside source implementations.

• Add IAssetSource interface (same methods as IAssetResolver)
• Add ResourcesAssetSource (default), AssetDatabaseAssetSource (editor with convention resolution via FindAssets), AddressableAssetSource
• Update generators to emit AssetSource.LoadAsset(path) directly
• Remove IAssetCatalog and DefaultAssetCatalog
• Deprecate IAssetResolver (extends IAssetSource for backward compat)
• UIComponent falls back to IAssetResolver if configured, for migration

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.59%. Comparing base (f633b5c) to head (39981c6).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ts/UIComponents/Editor/AssetDatabaseAssetSource.cs	97.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   94.58%   92.59%   -1.99%     
==========================================
  Files          60       62       +2     
  Lines        2068     2135      +67     
  Branches      288      287       -1     
==========================================
+ Hits         1956     1977      +21     
- Misses         95      141      +46     
  Partials       17       17              

Flag	Coverage Δ
automated	92.59% <98.83%> (-1.99%)	⬇️

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