[args] Add core OptionDescriptor types and ArgResults lookup (1/3) - #1012

…d bundles

- Add `package:args/typed.dart` (`OptionDescriptor`, `OptionBundle`, `ValueOptionBundle`, `CliEnum`, and typed extensions on `ArgResults`, `Command`, `ArgParser`, and `Option`).
- Add package-private `descriptorOf`, `isOptionRegistered`, and `rootOf` helpers in `lib/src/` to avoid external `Expando` registries, resolve `defaultsTo: null` option registration in `O(1)`, and automatically link subcommand `ArgResults` to top-level global results.

…nd add verboseOnly/override hooks

- Validate `IntOptionDescriptor` and `DefaultedIntOptionDescriptor` at parse time in `Parser._validateAllowed` so invalid integer arguments throw `ArgParserException` during `ArgParser.parse()` (and `UsageException` in `CommandRunner.run()`).
- Remove `{ArgResults? globalResults}` plumbing from `OptionDescriptor`, `ValueOptionBundle.parse`, and `SafeArgResults` since `Parser.parse()` automatically links child `ArgResults` to the root `ArgResults`.
- Hoist `aliases` onto `OptionDescriptor<T>` and extract `_StringOptionDescriptorBase<R>` and `_FlagOptionDescriptorBase<R>`.
- Add `verboseOnly` and `{bool verboseHelp = false, bool? hide, String? help}` registration overrides to `OptionDescriptor.addTo`, `OptionBundle.register`, and `ArgParserBundleExtension`.
- Widen bulk registration methods to `Iterable<...>` and add `addDescriptor` / `addDescriptors` on `ArgParser`.

…gh OptionBundle

- Add `OptionDescriptor.isRegisteredOnParser(ArgParser)` and check
  `identical(descriptorOf(option), this)` in `isRegistered`,
  `wasProvided`, `_resolveTargetResults`, and `getValue` so same-name
  collisions (e.g. an imperative flag or distinct descriptor sharing an
  option name) never falsely match or prevent `OptionScope.any` from
  falling back to `globalResults`.
- Update `CommandSafeArgResults.hasOption` to check `argParser` and
  `runner?.argParser` before `argResults` is parsed, as well as post-parse
  `argResults` / `_effectiveResults`.
- Pass `bool? hide` through `OptionBundle.register`, `registerOnCommand`,
  `CommandSafeArgResults.registerOptionBundle` / `registerOptionBundles`,
  and `ArgParserBundleExtension.registerBundle` / `registerBundles`.
- Default `MultiOptionDescriptor.defaultsTo` to `const <String>[]`.

…lts lookup (1/3)

Defer IntOptionDescriptor, EnumOptionDescriptor, CliEnum, OptionBundle,
Command extensions, and the public lib/typed.dart export to stacked PRs
2/3 and 3/3 so each slice can be reviewed independently.

- Expose `value(ArgResults)`, `valueOrDefault(ArgResults)`, `wasParsed(ArgResults)`, and `isRegistered(ArgResults)` directly on `OptionDescriptor<T>` with strict registration verification.
- Remove `OptionScope`, `ArgResults._root`, and `SafeArgResults` so typed descriptors layer directly on `ArgParser` and `ArgResults`.
- Shorten concrete descriptor names (`FlagDescriptor`, `NullableFlagDescriptor`, `StringDescriptor`, `DefaultedStringDescriptor`, `MultiStringDescriptor`), omit `defaultsTo` on nullable descriptors, and add `TypedArgParserExtension` / `TypedOptionExtension` in `typed_extensions.dart`.