cores: restrict connector fallback to arduino-header-r3 only

[soburi]

Motivation

• Prevent the connector auto-generation introduced earlier from matching arbitrary arduino_header nodes and narrow support to the Arduino Uno R3 connector format only, as required by the variant/support policy.

Description

• Add a compatibility gate DT_NODE_HAS_COMPAT(DT_NODELABEL(arduino_header), arduino_header_r3) to enable connector-derived pin generation only for R3-style headers.
• Emit a compile-time #error "Only arduino-header-r3 connector is supported" when an arduino_header node exists but is not R3-compatible to make the supported scope explicit.
• The change is localized to cores/arduino/Arduino.h and documentation/variants.md was intentionally left unchanged.

Testing

• Inspected the modified header to confirm the new DT_NODE_HAS_COMPAT guard and #error are present, and that inspection succeeded.
• Performed repository diffs and status checks to verify only cores/arduino/Arduino.h was modified and documentation/variants.md remained untouched, and these checks succeeded.
• Generated the PR metadata using the local tooling to summarize the change, and that operation completed successfully.

---

Codex Task

[Copilot]

Pull request overview

Restricts the Arduino connector-derived pin auto-generation path to only the Arduino Uno R3 header format, preventing unintended matches on arbitrary arduino_header connector nodes.

Changes:

• Gate ZARD_CONNECTOR auto-selection behind DT_NODE_HAS_COMPAT(..., arduino_header_r3).
• Emit a compile-time #error when an arduino_header node exists but is not R3-compatible (in the connector-fallback path).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.