Simplify the README

[lxsmnsyc]

The README carried a lot of prose that explained internals rather than usage. This trims it to what a reader needs, without dropping any documented API.

• Remove every em-dash. Each one became a period and a new sentence, or a list item.
• Turn multi-point paragraphs into lists. This covers what a target object accepts, what a component wrapped by Motion.create must do, and what the Playwright suite covers that jsdom cannot.
• Cut the rationale that does not change what a reader writes. The reduced motion justification, the FLIP and projection detail, and the frame loop reasoning behind motion values are each one sentence now.
• Fold the Introduction heading into a single line under the tagline.
• Replace literary phrasing with plain wording.
• Put the commands first in the Testing section and the port caveat last.
• Document pnpm run storybook in Examples. Storybook came back with the last upstream sync but the section only mentioned the playground.

Word count drops from 4047 to 3412. All 25 headings still resolve and no anchor is broken.