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/closures/expected/0302-closures-closures.stdout

@@ -0,0 +1,13 @@
+basic: 142
+snapshot: 15
+clamp: 50 0 100
+[INFO] system ready
+factory: 105 110
+auto-promote: 21
+hof: 40
+reduce: 15
+compose: 30
+multi: 105 500
+widget: slider = 42
+widget: no handler
+=== DONE ===