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/modules/0702-modules-c-import-ns.sx

@@ -0,0 +1,10 @@
+#import "modules/std.sx";
+tc :: #import "tests/fixtures/test_c.sx";
+
+main :: () -> i32 {
+    a := tc.add_numbers(10, 20);
+    b := tc.multiply(5, 6);
+    print("tc.add_numbers(10, 20) = {}\n", a);
+    print("tc.multiply(5, 6) = {}\n", b);
+    0
+}