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refactor: build Map from comprehension instead of mutating loop

Following the pattern in builtin/json.mbt (Map.to_json), apply the same
"build Map without mutation" idiom across three more sites:

- sorted_map/utils.mbt SortedMap::to_json
- immut/sorted_map/traits_impl.mbt SortedMap::to_json
- argparse/parser_lookup.mbt build_long_index

Each had `let m = Map([], capacity=...)` followed by an imperative
populating loop. Replaced with a single Map comprehension expression:

```diff
-pub impl[K : Show, V : ToJson] ToJson for SortedMap[K, V] with to_json(self) {
-  let capacity = self.length()
-  guard capacity != 0 else { return Json::object(Map([])) }
-  let jsons = Map([], capacity~)
-  self.each((k, v) => jsons[k.to_string()] = v.to_json())
-  Json::object(jsons)
+pub impl[K : Show, V : ToJson] ToJson for SortedMap[K, V] with to_json(self) {
+  Json::object(
+    Map(
+      capacity=self.length(),
+      [for k, v in self => (k.to_string(), v.to_json())],
+    ),
+  )
 }
```

`build_long_index` collapses the same way that `build_short_index`
already did (in #3522). The `guard capacity != 0` early-return is
removed because `Map::Map` already handles capacity=0 (it bumps to
`default_init_capacity`).

Style: labeled args (`capacity=...`) before positional, per maintainer
preference.

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

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

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