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/1206-ffi-medium-struct.c

@@ -0,0 +1,13 @@
+// Companion to examples/101-ffi-medium-struct.sx — a single
+// roundtrip through a 16-byte integer-only struct. Pinned in a
+// dedicated example because integer aggregates in this size class
+// route through emit_llvm.zig's `[2 x i64]` ABI coercion slot, a
+// different path from the small (≤8 B) integer struct, the 16-byte
+// HFA, and the >16 B byval-pointer cases.
+
+typedef struct { long long a; long long b; } Pair64;
+
+Pair64 ffi_pair64_swap(Pair64 p) {
+    Pair64 r = { p.b, p.a };
+    return r;
+}