feat(selfhost): add RS0021 (NON_EXHAUSTIVE_MATCH) to the rss checker

Port match-exhaustiveness (crate::analyzer::exhaustiveness, simplified for the
corpus) into selfhost/check.rss as the 17th parity code. A match statement or
expression is non-exhaustive unless a guardless arm set covers its scrutinee:
Some/None (Option), Ok/Err (Result), true/false (Bool), every variant of a
declared sum, an irrefutable pattern for a single-constructor tuple/struct, a
capping rest pattern for List, or a `_` wildcard. Malformed (no `=>`) and
guarded arms contribute no coverage.

Scrutinee root inference mirrors hir_expr_type_name: a let-bound local resolves
through its RHS (constructor / unqualified-fn return type), a param through its
declared type, qualified calls and `?`-unwrapped expressions fall back to the
builtin Some/None|Ok/Err rule. Copies find_top_sym/find_top_if/is_kw_text from
astdump; the arm walk reuses next_line_or_block_end.

Real sum decls are distinguished from a variable literally named `sum` (the
lexer tags every "sum" text with WORD_SUM) via a line-start/`pub`-guarded probe.

checker_parity_corpus: 619/619, 0 mismatches, 0 run-failures (17 codes).

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","RS0009","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","RS0021","RS0024","RS0028","RS0033"];
 
 fn is_target_code(code: &str) -> bool {
     CHECKER_TARGET_CODES.contains(&code)