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perf(argparse): thread ArrayView[String] through subcommand dispatch

Four `.to_owned()` calls in the parser hot path were eagerly
materializing an `Array[String]` from what downstream consumers accept
as `ArrayView[String]`:

- `default_argv()` returns the initial argv slice — `Command::parse`
  already declares its `argv?` parameter as `ArrayView[String]`.
- `parser.mbt` built two `rest` arrays (the help-subcommand path and
  the generic-subcommand dispatch path) only to pass them to
  `parse_command`, which also takes `ArrayView[String]`.
- `parser_lookup.mbt::split_long` was calling `to_owned()` on
  `ArrayView[StringView]` just to invoke `.join("=")`, which works
  directly on the view.

Returning / passing views saves one `Array` allocation per call. On a
program with nested subcommands, the dispatch path takes several of
these per invocation. `resolve_help_target` only needs pattern-match
and iteration over `argv`, which work on `ArrayView[String]` — changed
the private-signature's parameter type accordingly.

No mbti change: `default_argv` and `resolve_help_target` are private
`fn`s, and `parse_command` already accepted `ArrayView[String]`.

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