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/modules/expected/0713-modules-json-writer.stdout

@@ -0,0 +1,5 @@
+doc: {"name":"a\"b\n","tab":"x\ty","bs":"c\\d","ctrl":"\u0001","n":-7,"zero":0,"pos":7,"min":-9223372036854775808,"max":9223372036854775807,"ok":true,"nil":null,"xs":[1,-2,3],"nested":{"k":"v"}}
+buffer-exact: ok
+buffer-len: ok
+overflow-raised: ok
+file-exact: ok