moonbitlang / core / 4139

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refactor(argparse): view + range patterns in is_negative_number

Replace manual UTF-16 walk with a destructuring view pattern and a
character-range pattern for the digit check:

```
- guard arg.get_char(0) is Some('-') else { return false }
- for i = 1; i < arg.length(); {
-   let ch = arg.get_char(i).unwrap()
-   if ch < '0' || ch > '9' { return false }
-   continue i + ch.utf16_len()
- }
+ guard arg is ['-', .. rest] else { return false }
+ for ch in rest {
+   if ch is ('0'..='9') { continue } else { break false }
+ } nobreak { true }
```

Edge cases preserved by the leading `length() < 2` guard: "" and "-"
return false; "-0" returns true; "--5" / "--" return false.

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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