Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.
An `extern LIB "csym"` ref must name a declared #library / #import c unit,
like its `#foreign LIB` twin (example 1620). Today checkForeignRefs reads
only foreign_expr.library_ref and skips the extern keyword's extern_lib, so
a bogus `extern nosuchunit "abs"` compiles silently (the symbol resolves
via the default image and runs). Expected pins the DESIRED compile-time
diagnostic; the next commit extends checkForeignRefs to green it. Fourth
extern/#foreign divergence and a prerequisite for the fn-decl migration.
647 corpus (1231 xfail), 444 unit.
