[feature request] self-update

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no app markets help updating thorium apk.

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thorium to support built-in self-update capability.

**Additional Notes**
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The issue requests built-in self-update support for Thorium Android but names no files, tests, or entry points. Start by mapping how Android APK releases are currently distributed and define the supported update flow; done requires an agreed design and implementation scope for self-updating outside app markets.

Written by the indexing model from the issue text.