Multiplexr 0.5.1 preview 2

Plugin runtime and provider fixes for 0.5.1.

• Enables the isolated plugin runtime in normal builds.
• Fixes GitHub package downloads and network timeouts.
• Adds SDK 1.2 JSON POST, host-managed API-key headers and media-specific provider selection.
• Reorganizes extensions into searchable Installed and Browse views with setup guidance.

Install this APK before the 0.2.0 metadata plugins. The old 0.1.0 packages were starters without metadata handlers. In Plugins & themes, install a provider, review permissions, allow the intended collection, configure credentials if required and enable it.

TMDB needs an API Read Access Token. Open Library needs no key. Audible US is experimental. AudiobookDB remains gated pending written integration approval and has fixture tests only.

Validation: static analysis, Flutter regressions and phone layout tests, 227 native unit tests, 20 Android emulator instrumentation tests, and 31 SDK tests. Open Library and Audible endpoints returned live results; TMDB was fixture-tested without a supplied token. Physical-device verification remains outstanding.

Preview 2 also fixes reading-collection permission dialogs and release shrinking of the JNI host callback. Provider errors no longer count as interpreter crashes. Manage > Test lookup searches a granted collection without changing library records. A live Open Library search passed in the signed release APK on an Android emulator.