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-objc/1307-ffi-objc-expression-bodied-objc-method.sx

@@ -0,0 +1,20 @@
+// M1.0 (xfail) — '=>' expression-body form inside '#objc_class'
+// member methods.
+//
+// Today: parseRuntimeClassDecl ([src/parser.zig:1262]) accepts ';'
+// (declaration) or '{ ... }' (block body) but not '=>'. Trying
+// '=>' surfaces 'expected ;' at the arrow.
+//
+// Next commit extends the member parser to accept the arrow
+// form, mirroring the existing parseFnDecl ('=>') arm, and this
+// snapshot flips from a parser error to '42\n'.
+
+#import "modules/std.sx";
+
+SxFoo :: #objc_class("SxFoo") {
+    greet :: (self: *Self) -> i32 => 42;
+}
+
+main :: () -> i32 {
+    0
+}