Added example/example.dart and README.md (Features/Getting started/Usage/
Algorithm), recorded the manual accuracy spot-check findings in
docs/spec/README.md's Known limitations, and closed a coverage gap on
guess.dart's toString/==/hashCode overrides with a small dedicated test file.

make pre_commit is green (format, analyze, license_check via the real
addlicense binary, 74 tests) and make coverage reports 100% line coverage
(128/128) across every hand-written source file. dart pub publish --dry-run
reports 0 warnings and a correctly-scoped 67 KB archive.

Plan moved to docs/plans/completed/ with status Complete and a full summary.
Not pushed to GitHub, not published to pub.dev -- both are for the user to
do, per the agreed hand-off boundary.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>