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/basic/0014-basic-code.sx

@@ -0,0 +1,9 @@
+#import "modules/std.sx";
+
+generate::() -> string {
+    return "print(\"hello from the other side\n\");";
+}
+
+main :: () {
+    #insert generate();
+}