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-impl.sx

@@ -0,0 +1,10 @@
+// Helper that defines the impl. 179-impl-visibility's user file does NOT
+// directly import this — that's the whole point of the test.
+#import "modules/std.sx";
+#import "./0410-protocols-impl-visibility-types.sx";
+
+impl Into(Wrap) for i64 {
+  convert :: (self: i64) -> Wrap {
+    .{ v = self * 10 }
+  }
+}