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/diagnostics/expected/1155-diagnostics-for-not-iterable.stderr

@@ -0,0 +1,5 @@
+error: cannot iterate this expression — if the parens were call arguments, a call iterable also needs a capture (`for f(n) (x) { }`) or parentheses (`for (f(n)) { }`)
+  --> examples/diagnostics/1155-diagnostics-for-not-iterable.sx:11:9
+   |
+11 |     for f(n) { }
+   |         ^