refactor(grammar): use is_multiple_of and let-chains

Replace modulo operations with is_multiple_of calls and simplify nested if
statements using let-chains in the grammar generator.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

src/persistence/mod.rs

@@ -2232,7 +2232,7 @@ mod generator {
                     self.emit(alts[idx], out);
                 }
                 Expr::Opt(inner) => {
-                    if self.pick() % 3 != 0 {
+                    if !self.pick().is_multiple_of(3) {
                         // ~66% chance of emitting
                         self.emit(inner, out);
                     }