A test pins that a thought is read back only by its author: newest first, `since` respected, text exactly as kept

src/core/thoughts.ts says that a thought belongs to nobody else. No test calls recentThoughts or thoughtOf directly.

Done when test/thoughts-read.test.ts shows:

[ ] recentThoughts returns the author's thoughts newest first, cut at limit, and only those at or after since.
[ ] Another person's id gets none of them, and thoughtOf with a real thought id and the wrong author is undefined.
[ ] Text with markup, emoji and newlines comes back byte for byte.

Files: test/thoughts-read.test.ts, src/core/thoughts.ts

A test pins that a thought is read back only by its author: newest first, `since` respected, text exactly as kept #6715