Define and publish supported target matrix for bdk-dart

[reez]

We need an explicit, documented support matrix so users know what is production-
supported vs experimental, and so CI can validate the right targets.

Scope

• Define supported targets by platform + architecture (Android, iOS, macOS, Linux, Windows).
• Define support level per target (supported, experimental, not supported).
• Define runtime context per target (Dart CLI, Flutter).
• Define validation method per target (which CI job proves it).
• Document this in repo docs (README section and/or SUPPORTED_TARGETS.md).

Acceptance Criteria

• A target matrix is committed to the repo and linked from README.md.
• Each target has an explicit support level.
• Each supported target has a CI validation plan mapped to it.
• Requirements for production-ready release #16 checklist item is updated to link this issue.

Notes

• Web can be tracked separately; not required to close this issue unless explicitly made a blocker in Requirements for production-ready release #16.