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/ffi/1212-ffi-04-fp-struct.c

@@ -0,0 +1,29 @@
+#include "1212-ffi-04-fp-struct.h"
+
+FQuad ffi_fquad_make(float a, float b, float c, float d) {
+    FQuad r = { a, b, c, d };
+    return r;
+}
+
+FQuad ffi_fquad_reverse(FQuad v) {
+    FQuad r = { v.d, v.c, v.b, v.a };
+    return r;
+}
+
+float ffi_fquad_sum(FQuad v) {
+    return v.a + v.b + v.c + v.d;
+}
+
+DQuad ffi_dquad_make(double a, double b, double c, double d) {
+    DQuad r = { a, b, c, d };
+    return r;
+}
+
+DQuad ffi_dquad_reverse(DQuad v) {
+    DQuad r = { v.d, v.c, v.b, v.a };
+    return r;
+}
+
+double ffi_dquad_sum(DQuad v) {
+    return v.a + v.b + v.c + v.d;
+}