Add ESP32-C3 Opus loopback example

Software/UPSoftware/examples/OpusLoopbackEsp32C3/README.md

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+This sketch is a minimal ESP32-C3 Opus codec smoke test.
+
+What it does:
+- Generates a sine wave locally
+- Encodes it with Opus
+- Decodes it immediately again
+- Sends the decoded PCM to I2S
+
+Why this example:
+- It tests the raw Opus codec path without Ogg or CAN framing
+- It is closer to the later Pi -> CAN -> ESP32 decoder path than MP3
+- It keeps CPU load down by using 16 kHz mono and low encoder complexity
+
+Expected hardware:
+- ESP32-C3
+- I2S DAC / amp on:
+  - BCLK = GPIO 6
+  - LRCK = GPIO 7
+  - DIN = GPIO 5
+
+Required Arduino libraries:
+- `arduino-audio-tools`
+- `arduino-libopus`
+
+If this sketch runs and outputs a stable sine tone, the basic Opus encode/decode
+chain is working on the ESP32-C3.