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/ffi/expected/1211-ffi-03-large-struct.stdout

@@ -0,0 +1,6 @@
+big24 make    = (1, 2, 3)
+big24 rotate  = (3, 1, 2)
+big24 sum     = 6
+big48 make    = (10, 20, 30, 40, 50, 60)
+big48 reverse = (60, 50, 40, 30, 20, 10)
+big48 sum     = 210