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/errors/expected/1043-errors-lambda-raise-annotation-hint.stderr

@@ -0,0 +1,5 @@
+error: lambda body raises; declare its return type explicitly with `-> (T, !)` or `-> (T, !Named)`
+  --> examples/errors/1043-errors-lambda-raise-annotation-hint.sx:18:61
+   |
+18 |     print("{}\n", take(closure((x: i32) -> i32 { if x < 0 { raise error.Neg; } return x; }), -1));
+   |                                                             ^^^^^^^^^^^^^^^^