docs: make README robust to npm's renderer

[rohanpoudel2]

Follow-up to #13. npmjs.com sanitizes README HTML more aggressively than GitHub does, and for a published package the npm page is where most people meet the project. Two constructs in the new README depended on HTML surviving.

Changes

Dropped the <sup>-based footnote marker on --to. If npm strips <sup>, the dagger disappears and the note below the table loses its referent, leaving an orphaned sentence. It is now a plain bolded lead-in that reads correctly whether or not the tag renders.

Renamed "The --json contract" to "JSON output contract". Backticks in a heading slug to a triple dash (#the---json-contract), which is renderer-dependent. The plain heading is stable across GitHub, npm, and anything else.

Deliberately kept

<div align="center"> on the header and <sub> on the demo caption. If those tags are dropped the content still renders — left-aligned and full-size — so nothing is lost. They degrade rather than break.

Verification

• All 22 TOC anchors resolve against GitHub's slug algorithm, no duplicate slugs (checked with a script, not by eye).
• Both relative link targets, ./LICENSE and ./MIGRATION.md, exist in the repo — which is what npm rewrites relative links against, via the repository field.
• prettier --check passes.

Note, not addressed here

MIGRATION.md is not in the files array, so it is not in the published tarball. The npm page link still works (npm rewrites it to the GitHub repo), but the link is dead for anyone reading the README from node_modules. Adding it to files would fix that; left out of this PR since it changes published package contents.