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/protocols/0410-protocols-impl-visibility.sx

@@ -0,0 +1,21 @@
+// Impl visibility — an `impl Into(...) for ...` is registered into
+// the global impl table when its module is imported anywhere in the
+// program, but is only **visible** from files that themselves
+// transitively import the impl's defining module.
+//
+// Setup:
+//   - 179-impl-visibility-impl.sx declares an `impl Into(Wrap) for i64`.
+//   - 179-impl-visibility-user.sx tries `xx 7 : Wrap` but only
+//     imports the shared types — NOT the impl module.
+//   - The xx at the user-file site must produce a "no visible xx
+//     conversion" diagnostic, not silently fall through to whatever
+//     was registered in another module.
+//
+// The diagnostic is the success criterion — the compile error is the
+// expected output. Tests/expected/.txt captures it; .exit is 1.
+
+#import "modules/std.sx";
+#import "./0410-protocols-impl-visibility-impl.sx";
+#import "./0410-protocols-impl-visibility-user.sx";
+
+main :: () -> i32 { run_user() }