feat(selfhost-checker): add RS0009 (invalid pure effect)

A pure fn is flagged for a resource return, a mut/take param, a retains
item, or a body with/manage/non-pure-call. Non-pure-call resolution mirrors
the analyzer: type-qualified calls and constructors/enum-variants/pure-fns are
allowed; declared non-pure fns flag; unresolved names are ignored. Adds a
resource/sum-variant/pure-fn pre-pass. 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","RS0008","RS0010","RS0011","RS0012","RS0016","RS0017","RS0024","RS0028","RS0033"];
+const CHECKER_TARGET_CODES: &[&str] = &["RS0002","RS0003","RS0004","RS0005","RS0006","RS0007","RS0008","RS0009","RS0010","RS0011","RS0012","RS0016","RS0017","RS0024","RS0028","RS0033"];
 
 fn is_target_code(code: &str) -> bool {
     CHECKER_TARGET_CODES.contains(&code)