Document and test Remix host capability requirements

[leoafarias]

Problem

Remix is composable inside an application's existing host, but public examples and test helpers predominantly use MaterialApp and Scaffold. The infrastructure required by ordinary widgets, portal-based overlays, and route dialogs is not documented or protected as an explicit compatibility contract.

Scope

• Document a host capability matrix covering:
  • ordinary Remix/Fortal widgets in an existing subtree;
  • menu, select, popover, and tooltip with a caller-provided Overlay;
  • dialogs with a caller-provided Navigator;
  • Material, Cupertino, Widgets, and router-based application hosts.
• Add a WidgetsApp.builder test using Flutter's Overlay.wrap with no Navigator, and exercise the portal component families with Fortal/Mix inheritance.
• Add dialog tests under WidgetsApp with an explicitly caller-owned Navigator.
• Update README, documentation, examples, and shared test guidance so MaterialApp and Scaffold are presented as interoperability choices rather than Remix requirements.

Acceptance criteria

• Portal component tests pass with Overlay.wrap and no Navigator.
• Dialog tests pass with a caller-provided Navigator, and the requirement is documented.
• Public documentation contains a concise host capability matrix.
• No Remix-owned application, overlay-host, navigation, or scaffold abstraction is introduced.

Non-goals

• Rewriting the dashboard shell.
• Adding RemixApp, RemixOverlayHost, or RemixScaffold.
• Removing Material imports, default icons, or uses-material-design.
• Changing the dashboard toast without a current failing regression test.

Supersedes the focused host-contract portion of #82. The dashboard itself landed independently in #95.

References

• Flutter Overlay.wrap
• WidgetsApp.builder
• OverlayPortal

[leoafarias]

Host direction confirmed:

• Do not add RemixApp, RemixOverlayHost, or another required Remix host.
• Migrate packages/remix/example to WidgetsApp with a caller-owned Overlay.wrap; the canonical example should use neither MaterialApp nor Scaffold.
• Use a caller-owned Navigator only in dialog examples, because dialogs are routes.
• Keep Material, Cupertino, and router hosts as compatibility coverage, not default setup guidance.
• Keep the dashboard host/shell outside this issue; it is an interoperability showcase, not the minimal Remix host contract.