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refactor(argparse): use 'is' pattern for Some-or-noop match blocks

Multiple sites in argparse/parser_globals_merge.mbt followed the same
shape:

```moonbit
match X { Some(y) => action(y); None => () }
```

Replaced with the equivalent `is`-pattern:

```moonbit
if X is Some(y) { action(y) }
```

Plus:
- one early-Some-fallback `match parent_source { Some(s) => Some(s); None => child_source }` collapsed to `if parent_source is Some(_) { parent_source } else { child_source }`
- one big-block `match child.flags.get(name) { Some(v) => big-body; None => () }` flattened with `guard ... is Some(v) else { return }`
- nested `match Some/None` inside `propagate_globals_to_child` flattened to nested `if X is Some(...)`

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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