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/0053-basic-ufcs-opt-in.stdout

@@ -0,0 +1,8 @@
+marked: 42
+alias: 41
+direct: 41
+pipe: 41
+marked-direct: 42
+ptr-recv: 15
+generic-dot: 7
+generic-direct: 7