Move hardcoded error string in CompressAssemblies to Properties.Resources with XA5303 error code

[Copilot]

CompressAssemblies.cs uses a hardcoded Log.LogError() string for the DescriptorIndex parse failure, making it non-localizable and missing a proper error code.

Changes

• Properties/Resources.resx / Resources.Designer.cs: Add XA5303 resource string
• CompressAssemblies.cs: Replace Log.LogError(...) with Log.LogCodedError("XA5303", ...)
• Documentation/docs-mobile/messages/xa5303.md: New error documentation
• Documentation/docs-mobile/messages/index.md: Add XA5303 to index

// Before
Log.LogError ($"Failed to parse 'DescriptorIndex' metadata value '{descriptor_index_string}' for assembly '{assembly.ItemSpec}'");

// After
Log.LogCodedError ("XA5303", Properties.Resources.XA5303, descriptor_index_string, assembly.ItemSpec);