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/modules/1617-modules-library-nested-namespace/b.sx

@@ -0,0 +1,5 @@
+// Middle module: aliases c.sx, putting its #library one namespace deep.
+#import "modules/std.sx";
+c :: #import "c.sx";
+
+version_via_b :: () -> string { return c.pcap_version(); }

examples/modules/1617-modules-library-nested-namespace/c.sx

@@ -0,0 +1,11 @@
+// Innermost module: owns the #library and its extern fn.
+#import "modules/std.sx";
+
+pcaplib :: #library "pcap";
+pcap_lib_version :: () -> ?cstring extern pcaplib "pcap_lib_version";
+
+pcap_version :: () -> string {
+    p := pcap_lib_version();
+    if p == null { return ""; }
+    return from_cstring(p!);
+}