moonbitlang / core / 4157

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refactor(internal/regex_engine): for-in iteration in SlotBook::find_unused

Replace the c-style index loop in `SlotBook::find_unused` with `for i, word in self.0`,
which also caches the word once instead of indexing twice.

```diff
-  for i = 0; i < self.0.length(); i = i + 1 {
-    if self.0[i] != -1 {
-      let bit = self.0[i].lnot().ctz()
-      return Slot::from_index(i * 32 + bit)
+  for i, word in self.0 {
+    if word != -1 {
+      let bit = word.lnot().ctz()
+      return Slot::from_index(i * 32 + bit)
     }
   }
```

`find_unused` is called during regex *compilation* (per pattern, not per match),
so this is not a matcher hot path.

Semantics-preserving: `moon test` (6425/6425) passes and `moon info` produces
no `.mbti` change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

3 of 3 new or added lines in 1 file covered. (100.0%)

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