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/basic/expected/0050-basic-for-multi-iterable.stdout

@@ -0,0 +1,9 @@
+1:0 2:1 3:2 4:3 5:4 
+[0]=10 [1]=20 [2]=30 
+dot=300
+arrow-range s=6
+arrow-coll t=60
+v7 v8 v9 
+escape n=3
+1/10/100 2/20/101 3/30/102 4/40/103 
+after ref: 1 3 5 7