docs(remix): define host capability contract

[leoafarias]

Description

Defines Remix's host capability contract without introducing a Remix-owned app, overlay, navigator, or scaffold abstraction.

• Migrates the canonical package example to FortalScope + WidgetsApp + caller-owned Overlay.wrap, with no MaterialApp, Scaffold, or Navigator.
• Adds widget contracts for ordinary Fortal widgets, menu/select/popover/tooltip under an overlay without navigation, and dialogs under an explicit caller-owned navigator. Overlay content also verifies live Fortal and Mix inheritance.
• Aligns the README, documentation homepage, consumer skill, component guidance, shared test guidance, and docs validator with the verified host requirements.

Validation

• fvm flutter analyze — passed with no issues.
• fvm dart run melos run ci — passed generation, documentation, Fortal parity, 20 dashboard tests, and 2,115 Remix tests.

Related Issues

Closes #96.

---

Checklist

Note: Updating the pubspec.yaml and CHANGELOG.md is not required. These are handled automatically during the release process.

• My PR includes unit or integration tests for all changed/updated/fixed behaviors.
• I have updated or added relevant documentation (doc comments with ///).
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.