learn 2.3.0

• Choice items (learn-choice/1, src/tutor/choice.mjs). A recall question keys one answer and names the misconception behind each wrong choice. diagnoseChoice() tells a wrong attempt which misconception it matches, using the same top levels as the arithmetic tree, and never returns the keyed answer. doctor checks that with a probe and a known-bad item.
• A browser entry, @harperz9/learn/browser (src/browser.mjs). Practice, scheduling and diagnosis load in a page with no bundler. The session functions moved from tutor.mjs into session.mjs, which has no Node built-ins; tutor.mjs re-exports them unchanged. A test walks the entry's import graph and fails on any node: or bare import.
• An attempt may carry a misconception leaf, and misconceptions() counts it.