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refactor(argparse): collapse format_entries to fold + comprehension (#3550)

* refactor(argparse): collapse format_entries to fold + comprehension

`format_entries` had two stateful loops to (1) compute the maximum name
length, then (2) build the formatted entries. Replace both with a single
fold + comprehension:

```diff
 fn format_entries(display : Array[(String, String)]) -> Array[String] {
-  let entries = Array::new(capacity=display.length())
-  let max_len = for item in display; max_len = 0 {
-    let (name, _) = item
-    if name.length() > max_len {
-      continue name.length()
-    } else {
-      continue max_len
-    }
-  } nobreak {
-    max_len
-  }
-  for item in display {
-    let (name, doc) = item
-    let padding = " ".repeat(max_len - name.length() + 2)
-    entries.push("  \{name}\{padding}\{doc}")
-  }
-  entries
+  let max_len = display
+    .iter()
+    .fold(init=0, (acc, item) => acc.max(item.0.length()))
+  fn fmt(name : String, doc : String) -> String {
+    let padding = " ".repeat(max_len - name.length() + 2)
+    "  \{name}\{padding}\{doc}"
+  }
+  [for item in display => fmt(item.0, item.1)]
 }
```

The nested `fmt` helper exists because string interpolation can't contain
`"` inside `\{ }` so the `padding` `let` can't be inlined into a
comprehension body — it's a single-expression closure capturing
`max_len`.

Removes one `let mut`-style accumulator and one imperative-push loop.

Semantics-preserving: `moon test -p argparse` (225/225) passes and
`moon info` produces no `.mbti` change.

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

* refactor(argparse): drop redundant .iter() before Array::fold

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

---------

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

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