Add streaming README demo

[roodboi]

What changed

• add the provided terminal GIF near the top of the README
• show one streamed JSON value progressing through three complete snapshots
• clarify that default snapshot cadence follows source chunks

Why

The opening explains what Schema Stream does, but a visual and concrete before/after sequence make the progressive behavior understandable at a glance.

Validation

• inspected the source GIF and an extracted mid-animation frame
• bun run format
• bun run lint
• git diff --check