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/diagnostics/1151-diagnostics-for-open-first.sx

@@ -0,0 +1,8 @@
+// The FIRST iterable drives the loop, so it must be bounded; an open range
+// `a..` may only follow it.
+
+#import "modules/std.sx";
+
+main :: () {
+    for 0.. (i) { }
+}