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/diagnostics/1159-diagnostics-array-const-mixed-elements.sx

@@ -0,0 +1,12 @@
+// An untyped array-literal constant infers its element type from the
+// elements (ints -> i64; a numeric int/float mix promotes to f64). A
+// NON-NUMERIC mix has no unified element type — diagnosed, with the
+// annotation as the fix.
+
+#import "modules/std.sx";
+
+BAD :: .["alpha", 1];
+
+main :: () {
+    print("{}\n", BAD[0]);
+}