chore(selfhost): ratchet AST corpus floor to 609/613

let-else/if-let/protocols/tuples landed 592 -> 609 byte-exact (0 run-failures).
Remaining 4 mismatches: view statement, await-try, two closure-effect cases.

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

Author: olwang

crates/rsscript/src/selfhost_parity.rs

@@ -1522,7 +1522,7 @@ const AST_SAMPLE_MIN: usize = 6;
 /// Floor for `ast_parity_corpus` — the number of corpus files whose rss AST dump
 /// already matches the oracle byte-for-byte. Ratchets up as the producer's
 /// coverage grows; a drop signals a regression. (Full parity = files.len().)
-const AST_CORPUS_PARITY_FLOOR: usize = 592;
+const AST_CORPUS_PARITY_FLOOR: usize = 609;
 
 /// Step-2 measurement gate (ignored by default): how many corpus files the rss
 /// producer reproduces byte-for-byte. Not full parity yet — this ratchets a floor