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/1105-diagnostics-compile-error.sx

@@ -0,0 +1,14 @@
+// `compile_error(msg)` raises a build-time diagnostic at the call
+// site. Used by builder fns (e.g. `#insert build_block_convert(...)`)
+// to reject malformed pack shapes with a clear message rather than
+// silently producing wrong code.
+//
+// The diagnostic appears at the source position of the
+// `compile_error` call. Argument must be a string literal — runtime
+// expressions can't be reported as compile errors.
+
+#import "modules/std.sx";
+
+#run compile_error("intentional compile error from #run");
+
+main :: () { }