Test the semantic recall's edge cases (#478) #542

What

The semantic recall merged in #478 has a happy-path test, but its edges — CJK with no word split, a stem matching across a word's endings, determinism, an empty query, and a [[topic]] graph with a cycle or a self-link — are not pinned. A later change could regress them silently.

Expected behavior

• Tests lock in: a Chinese query finds a Chinese entry; a word stem matches across endings; the same index+query ranks the same every time; an empty query returns nothing; a two-way [[topic]] link does not loop and a self-link is dropped; an unknown start and a topic with no link are empty.

Change scope

New core/test/memory-search-edge.test.js. Test only.

Why

Reliability: the fresh recall keeps working as advertised while it is still new.