feat: add initial ATmega328P HAL support

[nursude355]

Summary

Adds initial Embassy-compatible HAL support for the ATmega328P.

The implementation currently includes:

• GPIO for ports B, C and D
• Timer1-based Embassy time driver
• blocking ADC, I2C, SPI and USART drivers
• Timer0 and Timer2 PWM
• bit-banged 1-Wire support
• ATmega328P examples, including async blinky
• 8 MHz and 16 MHz clock configurations

Current validation

• Host cargo check passes
• Clippy with warnings denied passes
• All AVR examples pass cargo check for avr-none
• 8 MHz and 16 MHz configurations compile

Remaining validation

• Final linking with avr-gcc
• Flashing and testing on real ATmega328P hardware
• Peripheral hardware validation

This draft PR is also intended to request feedback on whether the HAL
should remain ATmega328P-specific or evolve into a broader embassy-avr
crate.

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[leftger]

I don't want to be a downer but the 328P is a significant departure from what Embassy currently supports. Cortex-M/Cortex-R/RISC-V are the common architectures. The 328P with 32KB of flash is absolutely tiny compared to what current microcontrollers offer. It's a 17-year-old device with decreasing use in industry. From a maintenance standpoint the ROI of bringing this into Embassy just isn't there. First and foremost probe-rs support for the ATMega328 is nonexistent. I'd consider debugger/flasher support before bringing this into Embassy.

[joelsa]

I'd also like to point to the following issues:

• AVR examples? #3754
• Compatibility with ATMEGA architecture #2453
• AVR support? #570

the existing naming schemes could give a hint towards answering this question:

This draft PR is also intended to request feedback on whether the HAL
should remain ATmega328P-specific or evolve into a broader embassy-avr
crate.