Stream Events code improvements

[x2d7]

Summary

Refactor event types for consistency and add JSON serialization support.

Refactor: rename events and unexport GetType (#39, #40)

Event types renamed for idiomatic Go style — EventNewToken → EventToken, EventNewToolCall → EventToolCall etc. New was redundant in type names. Deprecated aliases added for all renamed types, to be removed in v0.4.0.

StreamEvent.GetType is now unexported — the interface is sealed, external implementation was never intended.

eventType switched from uint/iota to string constants, making the type readable and serialization-friendly.

Feat: JSON serialization for events (#39)

Added MarshalEvent and UnmarshalEvent to serial.go. Each event is wrapped in an envelope with a type discriminator:

{"type": "token", "payload": {"text": "hello"}}

serial.go is intended as the home for all future serialization utilities — Messages, Client, Chat and others.

Closes #39
Closes #40