Fix: Remove broken on_stdout callback

[wasdhjklxyz]

Fixes #1

Problem

Videos don't render - blank output with continuous buffer modification errors.

Cause

The on_stdout callback tries to write to terminal buffers, which are non-modifiable and don't use stdout callbacks.

Solution

Removed the callback - termopen() handles terminal rendering automatically.

Environment

NVIM v0.11.3
Build type: Release
LuaJIT 2.1.1741730670