feat(selfhost-checker): add RS0024 (unknown type)

Recursive TypeRef validation over fields, params, and return types with
generic-param scope. Known set = 45 hardcoded builtins + the struct/resource
Item::Type names preloaded from CORE + STANDARD package .rssi interfaces
(mirrors hir.type_info). Corpus-green over 619 files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang

crates/rsscript/src/selfhost_parity.rs

@@ -503,7 +503,7 @@ fn parser_parity_corpus() {
 // ---------------------------------------------------------------------------
 
 /// Diagnostic codes the rss checker is expected to reproduce.
-const CHECKER_TARGET_CODES: &[&str] = &["RS0002","RS0003","RS0004","RS0005","RS0006","RS0007","RS0010","RS0011","RS0012","RS0016","RS0017","RS0028","RS0033"];
+const CHECKER_TARGET_CODES: &[&str] = &["RS0002","RS0003","RS0004","RS0005","RS0006","RS0007","RS0010","RS0011","RS0012","RS0016","RS0017","RS0024","RS0028","RS0033"];
 
 fn is_target_code(code: &str) -> bool {
     CHECKER_TARGET_CODES.contains(&code)