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/protocols/expected/0401-protocols-protocol-in-wrapper-struct.stdout

@@ -0,0 +1,4 @@
+inside add: 42
+inside add: 99
+items[0] = 42 (expected 42)
+items[1] = 99 (expected 99)