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/0015-basic-demo.sx

@@ -0,0 +1,15 @@
+std :: #import "modules/std.sx";
+
+vec3 :: (x:f32, y:f32, z:f32) -> std.Vector(3, f32) {
+    .[x,y,z]
+}
+
+main :: () {
+    v1 := vec3(1,0,0);
+    v2 := vec3(0,0,1);
+    s := 0.5;
+
+    sum := (v1 - v2);// math.cross(v1, v2);
+
+    std.print("{}\n", sum);
+}