Split ISerializer into its own file

[Copilot]

Extracts ISerializer and related interfaces into a dedicated file to match the existing pattern where IDeserializer has its own file.

Changes

New file: ISerializer.cs

• ISerializer - Core serializer interface
• ISerializeExt - Extension methods for ISerializer
• ITypeSerializer - Type serializer interface for non-primitive types
• ITypeSerializerExt - Extension methods for ITypeSerializer

Updated: ISerialize.cs

• Removed serializer interfaces (moved to ISerializer.cs)
• Retained serialization logic interfaces: ISerialize<T>, ITypeSerialize<T>, and provider types
• Removed unused imports (System.Diagnostics, System.Runtime.CompilerServices)

This establishes symmetry with the deserialization side where IDeserialize.cs contains logic interfaces and IDeserializer.cs contains deserializer interfaces.

Original prompt

Can you split ISerializer into its own file, like IDeserializer?

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