Gearslip: phone-side Android Auto client

README.md

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+<img src="docs/icon/gearslip-icon.png" alt="Gearslip icon" width="96" height="96">
+
+# Gearslip
+
+Bring any app to your car screen.
+
+## What this is
+
+Gearslip is a phone-side Android Auto client. It speaks the Android Auto protocol directly
+to a car head unit over USB and acts as a **Car App Library host**: apps that ship an
+Android Auto car UI (navigation and media apps) hand Gearslip a template, and Gearslip
+renders it natively on the car screen, full quality, no phone-screen mirroring. A separate
+screen-sharing mode is included for apps that don't ship a car UI at all.
+
+Built by reverse-engineering the protocol for interoperability - getting a non-Google client
+talking to a car head unit that only expects Google's own app.
+
+## Status
+
+Real progress, with one hard limit that's worth knowing before you build on this:
+
+- **Templated app hosting works.** All 16 template types the Car App Library defines are
+  rendered, including a keyboard Gearslip draws itself for search and sign-in. Verified
+  against several real navigation and media apps.
+- **Media playback works.** Car-enabled media apps are browsable and controllable from the
+  car UI, with lyrics and playback state, over the same host connection.
+- **The connection to a real head unit needs a certificate Google controls, and Gearslip
+  doesn't ship one.** A head unit only accepts a phone identity that chains to Google's
+  Automotive Link certificate authority - something only Google issues. Without one, Gearslip
+  still runs and renders everything correctly against a debug harness, but a real car will
+  reject the connection outright. See [SPIKE_FINDINGS.md](SPIKE_FINDINGS.md) for what was
+  tested, including one publicly-known leaked certificate that works on some older head
+  units and is rejected by newer firmware and by Google's own reference tooling.
+- **No adb, root, or Shizuku is required to run it.** Every feature is gated on an ordinary
+  Android permission or an on-device consent dialog - nothing needs privileged access, on
+  the phone or in the car.
+
+## Building
+
+```
+./gradlew :gearslip:assembleDebug
+```
+
+The APK lands at `build/Gearslip-debug.apk`.
+
+## Further reading
+
+- [SPIKE_FINDINGS.md](SPIKE_FINDINGS.md) - the certificate investigation in full: what was
+  tried, what a real head unit does and doesn't accept, and why.
+- [CUSTOM_HEADUNIT_SCOPING.md](CUSTOM_HEADUNIT_SCOPING.md) - the original scoping of what a
+  custom head unit client would need.
+
+## License
+
+AGPL-3.0 - see [LICENSE](LICENSE). A separate commercial license is available for private
+use outside the AGPL's terms; see [NOTICE.md](NOTICE.md).