RP: add pio program to drive bipolar steppers through driver board

[1-rafael-1]

This PR adds a pio program step_dirthat can be used to drive a 4-wire bipolar stepper motor using a driver board. It includes one example (duplicated for rp and rp23).

The pio program has three modes, depending on how low a frequency one needs for driving the motor. It uses two gpio, one is just the direction control and uses normal gpio toggling and the other is driven by a pio program to generate the pulses.

This PR has been coded with AI assistance. I have undertaken these following steps to verify the viability of this contribution:

• I have reviewed the code
• I have used a logic analyzer to confirm expected outputs
• I have tested on a rp2350 board, tmc2209 stepper driver board and nema17 bipolar 4wire motor.
• I have no rp2040 on hand so for this I assume parity.

Here is the LA output for 10 dedicated steps at 2Khz frequency using default setup. See pulses and measurements: this should work as expected.

Here is the test setup with hardware, working as expected:

I am happy to change/adapt code if concerns arise :-). I am equally happy to run/demonstrate other tests if requested.

[Copilot]

Pull request overview

This PR introduces a new step_dir PIO program/driver in embassy-rp for generating STEP pulses via PIO plus a software-controlled DIR GPIO, along with duplicated usage examples for RP2040 (rp) and RP235x (rp235x) boards.

Changes:

• Added embassy_rp::pio_programs::step_dir with configurable STEP pulse timing and frequency control.
• Added pio_step_dir.rs examples for both rp and rp235x.
• Exported the new module from embassy-rp/src/pio_programs/mod.rs.

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 2 comments.

File	Description
examples/rp235x/src/bin/pio_step_dir.rs	Adds an rp235x example demonstrating the new step/dir driver usage.
examples/rp/src/bin/pio_step_dir.rs	Adds an rp2040 example demonstrating the new step/dir driver usage.
embassy-rp/src/pio_programs/step_dir.rs	Implements the step/dir PIO program wrapper and API (timing, frequency, move, stop).
embassy-rp/src/pio_programs/mod.rs	Exposes the new step_dir module.

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