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/1185-diagnostics-weld-fn-runtime-call.sx

@@ -0,0 +1,16 @@
+// Diagnostic: a welded `compiler`-library function is comptime-only — it has no
+// runtime symbol (the comptime interpreter dispatches it to a Zig handler).
+// Calling one from runtime code is a build error with a clear message, NOT an
+// undefined-symbol link failure. (A comptime use — inside `#run` or a `::` —
+// is fine; see examples/0626.)
+
+#import "modules/std.sx";
+
+
+StringId :: u32;
+intern :: (s: string) -> StringId abi(.compiler);
+
+main :: () {
+    id := intern("called at runtime");
+    print("{}\n", id);
+}