docs: add compatibility guide

Author: SayanthRock

docs/COMPATIBILITY.md

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+# Compatibility Guide
+
+This project targets OPPO, OnePlus, and realme devices using ColorOS, OxygenOS, or realme UI.
+
+## Android versions
+
+| Android version | Status | Notes |
+|---|---|---|
+| Android 15 | Supported target | Needs testing across each brand skin |
+| Android 16 | Supported target | Expect stronger storage and system restrictions |
+| Android 17 | Experimental target | Test before customer release |
+
+## Brand support
+
+| Brand | Expected behavior |
+|---|---|
+| OPPO | Best target for ColorOS theme testing |
+| realme | Similar path expectations, but realme UI changes by region |
+| OnePlus | OxygenOS/ColorOS base differs by device and region |
+
+## Customer test checklist
+
+Ask every tester to send:
+
+- Device brand
+- Device model
+- Android version
+- ColorOS/OxygenOS/realme UI version
+- Root solution: Magisk, KernelSU, APatch, or none
+- Boot success after install
+- Theme folder detected or not
+- Screenshots of lock screen, home screen, wallpaper, and About phone
+
+## Safe compatibility rule
+
+Do not promise full theme support before testing on that exact model and firmware. OEM theme paths can change between updates, regions, and Android versions.