docs: add problem solver matrix

Author: SayanthRock

docs/PROBLEM_SOLVER_MATRIX.md

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+# Problem Solver Matrix
+
+Use this as the customer support brain of the module.
+
+## Main rule
+
+The module should give customers the best available option for their device. It should not pretend that every phone supports every feature automatically.
+
+## Problem matrix
+
+| Customer problem | Module answer | Status |
+|---|---|---|
+| Need default theme | Use Default Rock Premium theme pack | Starter ready |
+| Need home wallpaper | Apply through wallpaper settings | Safe |
+| Need lock wallpaper | Apply where device supports it | Safe where supported |
+| Theme not showing | Check device model, ROM version, and theme folder behavior | Test required |
+| Icons not changing | Use launcher or OEM theme support | Test required |
+| Sounds not changing | Use manual sound settings where available | Test required |
+| Phone feels slow | Use lag checklist and reduce heavy assets | Safe |
+| Boot feels slow | Keep module service lightweight | Safe |
+| Customer confused | Use support report template | Safe |
+| Device unsupported | Mark as not supported until tested | Safe |
+| LSPosed requested | Mark experimental and test first | Experimental |
+
+## Customer status labels
+
+- Works
+- Partial
+- Needs manual setup
+- Needs root
+- Needs device test
+- Not supported
+- Experimental
+
+## Best customer promise
+
+Give customers clear choices, tested device support, rollback help, and fast issue reporting.