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/closures/0301-closures-fn-pointers.sx

@@ -0,0 +1,28 @@
+#import "modules/std.sx";
+
+add :: (a: i32, b: i32) -> i32 { a + b }
+mul :: (a: i32, b: i32) -> i32 { a * b }
+
+apply :: (f: (i32, i32) -> i32, x: i32, y: i32) -> i32 {
+    f(x, y)
+}
+
+main :: () {
+    // Store function in variable
+    fp : (i32, i32) -> i32 = add;
+    print("fp(3,4) = {}\n", fp(3, 4));
+
+    // Reassign to different function
+    fp = mul;
+    print("fp(3,4) = {}\n", fp(3, 4));
+
+    // Pass function pointer as argument
+    print("apply(add,5,6) = {}\n", apply(add, 5, 6));
+    print("apply(mul,5,6) = {}\n", apply(mul, 5, 6));
+}
+
+// ** stdout **
+//fp(3,4) = 7
+//fp(3,4) = 12
+//apply(add,5,6) = 11
+//apply(mul,5,6) = 30