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/1190-diagnostics-protocol-missing-receiver.sx

@@ -0,0 +1,12 @@
+// A protocol method that omits the explicit receiver (the old implicit form)
+// is now a parse error — the receiver `self: *Self`/`self: Self` is required as
+// the first parameter. This guards the diagnostic.
+#import "modules/std.sx";
+
+Show :: protocol {
+    show :: () -> string;        // ERROR: missing `self` receiver
+}
+
+main :: () -> i32 {
+    return 0;
+}