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refactor(argparse): collapse usage_tail to filter-and-join

`usage_tail` had a `let mut tail = ""` plus four conditional `tail =
"\{tail} \{...}"` accumulations. Replace with a single array literal,
filter-non-empty, and join:

```diff
 fn usage_tail(cmd : Command) -> String {
-  let mut tail = ""
-  let required_options = required_option_usage(cmd)
-  if required_options != "" {
-    tail = "\{tail} \{required_options}"
-  }
-  if has_options(cmd) {
-    tail = "\{tail} [options]"
-  }
-  let pos = positional_usage(cmd)
-  if pos != "" {
-    tail = "\{tail} \{pos}"
-  }
-  let sub = subcommand_usage(cmd)
-  if sub != "" {
-    tail = "\{tail} \{sub}"
-  }
-  tail
+  let parts = [
+    required_option_usage(cmd),
+    if has_options(cmd) { "[options]" } else { "" },
+    positional_usage(cmd),
+    subcommand_usage(cmd),
+  ].filter(p => p != "")
+  if parts.is_empty() {
+    ""
+  } else {
+    " " + parts.join(" ")
+  }
 }
```

Eliminates the `let mut` accumulator. Each helper still called once.

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>

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