test: group examples into per-category folders

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.

examples/diagnostics/1176-diagnostics-import-parse-error-location/broken.sx

@@ -0,0 +1,6 @@
+// Deliberately broken: exercises import parse-error LOCATION reporting.
+// These leading comment lines push the parse error down so its line number
+// differs from the importer's — a mislocated caret would point here-or-wrong
+// instead of at the real offending token below.
+//
+broken :: 1 2;