v2: Documentation kept complete as v2 ships

[loyality7]

Context

v1 shipped with full KDoc coverage (zero Dokka "Undocumented:" warnings, enforced via reportUndocumented=true) and a published GitHub Pages site. That standard should not regress as v2 adds surface area — this issue is the tracking point for "docs are done" as a real, closeable v2 criterion, not an afterthought.

Scope

• Every new public class/method/property added by any v2 item ships with KDoc in the same PR, not as followup debt.
• ./gradlew :headless:dokkaHtml stays at zero undocumented warnings throughout v2.
• README updated per shipped v2 feature (a new capability with no README mention is not done).
• Dokka/GitHub Pages site stays current — it redeploys automatically on merge to main already (.github/workflows/docs.yml), so this is mostly a discipline check, not new infra.

Acceptance criteria

• At v2 close: dokkaHtml reports zero undocumented warnings
• Every v2 issue's PR included doc updates, none deferred to "docs later"
• README's feature list matches what's actually shipped
• Pages site reflects the final v2 API surface