In MinifyRenamer, avoid generating names for slots where the symbol
use count is zero. These represent declared symbols that are never
actually used (e.g., tree-shaken away or declared but not referenced).

We filter out zero-count slots before adding to the sorted array,
avoiding unnecessary sorting and iteration overhead. This is
particularly beneficial when nested scopes are tree-shaken, as their
pre-allocated symbol slots would otherwise still be processed.

Ported from oxc: oxc-project/oxc#18183

Co-Authored-By: Claude Opus 4.5 <[email protected]>

…uency slots (#18225)

#18183 introduced a bug when removing empty frequency slots. The code used `truncate` after sorting:

```rust
frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
if let Some(idx) = frequencies.iter().position(|x| x.symbol_ids.is_empty()) {
    frequencies.truncate(idx);
}
```

The problem is that `sort_unstable_by_key` doesn't guarantee order for items with equal keys. When multiple slots have `frequency: 0`, some have symbols (that need renaming but have no references), and some are empty. Since the sort order is unstable, an empty slot could appear **before** a non-empty slot, and `truncate` would incorrectly remove valid slots.

This fix uses `retain` instead, and moves it before the sort for better efficiency (sorting fewer elements):

```rust
frequencies.retain(|x| !x.symbol_ids.is_empty());
frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
```

Note for reviewer: I've locally tested in the `monitor-oxc`; there is no naming collision after this.