examples/comp: add comparator example tool

[daniel-p-carvalho]

Summary

Adds a dedicated command-line tool examples/comp for testing and interacting with analog comparator devices (/dev/comp*) from NSH.

Features:

• Read comparator state: comp read
• Enable/disable comparator via IOCTL: comp enable, comp disable
• Automated smoke test: comp test
• Optional dynamic analog threshold test via DAC voltage ramp: comp ramp (when CONFIG_DAC is enabled).
• Fully portable across architectures (pure POSIX / character driver interface).

Impact

• Provides an official example tool for the comparator subsystem in NuttX (similar to examples/adc and examples/dac).

Testing

• Host OS: Linux x86_64
• Target Board: STMicroelectronics NUCLEO-G431KB (STM32G431KB)
• Configuration: nucleo-g431kb:comp
• Toolchain: arm-none-eabi-gcc 13.2.1

Execution Logs on Physical Hardware:

1. Static state and IOCTL test:

nsh> comp
Running comparator test on /dev/comp0...
  1. Initial output state: 0
  2. ANIOC_COMP_DISABLE: OK
  3. Output state when disabled: 0
  4. ANIOC_COMP_ENABLE: OK
  5. Output state when enabled: 1
Comparator test PASSED on /dev/comp0

2. Voltage ramp test against internal DAC3-CH2:

nsh> comp ramp
=== Comparator Ramp Test ===
  COMP device : /dev/comp0
  DAC device  : /dev/dac5
  Sweep range : 0 -> 4095 (step 100, delay 5 ms)

     DAC=   0 (~   0 mV) | COMP output: 1
     DAC= 500 (~ 402 mV) | COMP output: 1
     DAC=1000 (~ 805 mV) | COMP output: 1
  -> [TRIP EVENT #1] DAC=1400 (~1128 mV) | COMP output: 1 -> 0
     DAC=1500 (~1208 mV) | COMP output: 0
     DAC=2000 (~1611 mV) | COMP output: 0
     DAC=2500 (~2014 mV) | COMP output: 0
     DAC=3000 (~2417 mV) | COMP output: 0
     DAC=3500 (~2820 mV) | COMP output: 0
     DAC=4000 (~3223 mV) | COMP output: 0

=== Test Summary ===
SUCCESS: Detected comparator transition(s)!

• Grounded input check: With INP grounded (0V), COMP output stayed at 0 throughout full DAC ramp.
• High-rail input check: With INP connected to 3.3V, COMP output stayed at 1 until DAC reached 3.14V (~3900), tripping as expected.
• Verified with checkpatch.sh: 0 errors, 0 warnings.

[daniel-p-carvalho]

Companion OS/driver PR in nuttx: apache/nuttx#20149