I2S: full duplex, auto rx sample rate

[Staars]

Description:

Key changes:

• Properly enables full-duplex I2S (simultaneous TX+RX on one I2S channel) which is used on most ESP32-P4 boards with a codec like the ES8311.

• For microphone recordings: automatic fallback to a compatible sampling rate

Smaller fixes and cosmetic changes:

• Rename sys.duplex → sys.full_duplex; makes intent clearer
• Fix I2SPrepareRx() returning wrong error code
• mic_stop marked volatile
• Remove duplicate break in Xdrv42() FUNC_LOOP

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.10
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass