README: fix stale claims, drop fluff

README: fix stale claims, drop fluff

- Drop the invented '★ top mark' — leaf classes only get a gold border
  (matching the viewer legend); reword to 'leaf', laid out at the top.
- Data-family constructors/field names ARE shown now: say so in the family
  view + extraction notes, and remove the stale 'no data-family RHS bodies'
  limitation.
- Update the demo HTML size (~780 KB -> ~840 KB, typecore.js now embedded)
  and drop 'ready to be explored'.
- Bump the example unit-ids 0.1.0.0 -> 0.2.1.0 and note the helper reads
  the id from the store as well as dist-newstyle.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>