feat: add MediaElement MVP control and initial tests

[lustsazeus-lab]

Summary

• add a new MediaElement control scaffold with dependency properties and playback state transitions
• wire MediaElement into automation peer control-type mapping
• add initial tests for null-source failure handling, deterministic state transitions, and XAML instantiation

Notes

• this PR intentionally delivers the MVP control contract first so the LibVLC backend/distribution path can be layered incrementally
• in this environment, dotnet is unavailable, so CI/local build confirmation should run in project CI or a .NET-enabled machine

Closes #5

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Remove escaped quotes from raw XAML literal

This test uses a C# raw string ("""), so \" is emitted literally into the XML; that produces markup like xmlns=\"urn:inkkslinger-ui\", which is not well-formed and will fail in XamlLoader.LoadFromString when it calls XDocument.Parse. As written, XamlLoader_CanInstantiate_MediaElement will throw before assertions and won't validate MediaElement instantiation.

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[lustsazeus-lab]

Addressed in commit 174b0a8 — removed escaped quotes in the C# raw string literal so the test now emits valid XML for XamlLoader.LoadFromString.