docs: scaffold polisher-control foundation

firmware/teensy/README.md

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+# Teensy Firmware Scaffold
+
+Suggested starting point: Teensy 4.1 using PlatformIO/Arduino framework.
+
+Core modules to implement:
+
+- protocol framing + CRC;
+- message decode/encode;
+- state machine mirror / safety state;
+- KWR75B-CAN decoder via isolated CAN transceiver;
+- table and arm encoder readers;
+- force PID and actuator output;
+- ODrive command/telemetry path;
+- telemetry packet builder;
+- fast interlocks and watchdogs.
+
+The included `src/main.cpp` is intentionally minimal. Replace with real modules as hardware decisions close.