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/generics/0200-generics-generic.sx

@@ -0,0 +1,17 @@
+#import "modules/std.sx";
+sum :: (a:$T, b:T) -> T {
+    return a + b;
+}
+
+main :: () {
+    x:=sum(2,3);
+
+    print("sum: {}\n", x);
+    print("sum: {}\n", sum(40,2));
+    print("sum: {}\n", sum(40,2.5));
+}
+
+// ** stdout **
+// sum: 42
+// sum: 42.500000
+//