Expand digital/PWM support, add validation and robust SCPI parsing, update README

[GiuseppeLeviBo]

Motivation

• Add support for additional digital pins (including LED_BUILTIN) and make PWM/servo handling more robust and safe.
• Prevent invalid channel usage by centralizing validation and parsing logic for analog/digital/PWM channels.
• Improve SCPI command error handling and make commands accept both SCPI indices and real Arduino pin numbers.

Description

• Extend digitalPins to include D2..D10 and LED_BUILTIN and compute digitalPinCount and pwmPinCount dynamically.
• Introduce utility helpers (isValidAnalogChannel, isValidDigitalChannel, isValidPwmChannel, findPwmChannelByPin, parseChannel, resolveDigitalChannel) to validate and parse channel arguments.
• Harden processCommand by validating inputs, returning ERR on invalid parameters, and supporting pin-number addressing for DIG:OUT queries and commands.
• Improve PWM and servo interplay by tracking pwmValue and servoAttached, detaching servos before setting PWM, and attaching servos only when SOUR:SERVO is used.
• Revise resetDevice to correctly reset/detach servos, clear PWM and digital states, and use the new counts; bumped identification string version to 1.2.
• Update READ?, ROUT:SCAN?, MEAS:VOLT?, MEAS:RAW? and related parsing to use the new validation and parsing helpers.
• Update README.md to document the expanded digital pin mapping, new examples showing DIG:OUT with pin numbers, and clarify PWM pins behavior.

Testing

• Built the Arduino sketch with the Arduino CLI for avr:uno to verify it compiles successfully.
• No automated unit tests are present in the repository, so behavior was enforced via compile-time checks only.

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[chatgpt-codex-connector]

Prefer physical pin matches before treating <ch> as index

resolveDigitalChannel accepts any token in 0..9 as a SCPI index before it ever checks digitalPins, so the new pin-number addressing described in README.md:155-157 is wrong for overlapping values. On Uno, DIG:OUT 2,1 drives digitalPins[2] (D4) instead of physical D2, and DIG:OUT 9,1 hits digitalPins[9] (D13) instead of D9. As written, physical D2-D9 can never be addressed even though this commit advertises that they can.

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[chatgpt-codex-connector]

Synchronize DIG:OUT? state when PWM drives D9/D10

Now that D9/D10 are exposed through the digital channel map, SOUR:PWM also needs to keep digitalState consistent. This block only updates pwmValue and the hardware pin, so after SOUR:PWM 0,255 a subsequent DIG:OUT? 7 still returns the stale boolean from the last DIG:OUT call (typically 0 after reset) even though D9 is being held HIGH. The shared digital/PWM behavior documented in README.md:157-163 therefore does not round-trip for the PWM pins.

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