fix(platforms): drop toxic ARDUINO_USB_CDC_ON_BOOT / USB_MODE flags

[zackees]

Summary

Drop -DARDUINO_USB_CDC_ON_BOOT=1 and -DARDUINO_USB_MODE=1 from the ESP32 board envs that were carrying them.

• esp32c6: removed both flags from build_flags
• esp32p4: removed ARDUINO_USB_MODE=1 (originally added per the Add ESP32 PARLIO-capable boards (C5, C6, H2, P4) with required ISR-IRAM flags #9 spec)
• esp32c3: removed the explicit =0 stanza — default is already safe, and the surrounding comment had the two states reversed

Each affected ini also gets a short "Do NOT add" comment so the flag doesn't silently come back in future PRs.

Why this matters

With ARDUINO_USB_CDC_ON_BOOT=1, the Arduino-ESP32 core routes Serial to the USB-CDC interface, which only enumerates when a USB host is connected. Boards powered from a wall adapter then hang in setup() the moment user code touches Serial.

This is exactly the failure mode a starter repo must not ship with: users copy these envs as their starting point, the fixture works on the bench (plugged into a PC), and then stops working the moment they deploy it on a wall adapter.

Test plan

• pio run -c platforms/platformio.esp32c3.ini still succeeds
• pio run -c platforms/platformio.esp32c6.ini still succeeds
• pio run -c platforms/platformio.esp32p4.ini still succeeds
• Confirm a C6 sketch with Serial.begin(115200); Serial.println("hi"); in setup() now boots on a wall adapter (no host connected)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved stability for ESP32 C3, C6, and P4 boards by adjusting default serial communication settings, preventing potential hangs during initialization when devices are powered without a USB host connection.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 85792954-7aa9-4722-a3d3-17802f47bf4e

📥 Commits

Reviewing files that changed from the base of the PR and between f04558e and e9e80ba.

📒 Files selected for processing (3)

• platforms/platformio.esp32c3.ini
• platforms/platformio.esp32c6.ini
• platforms/platformio.esp32p4.ini

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📝 Walkthrough

Walkthrough

USB CDC compile-time build flags (ARDUINO_USB_CDC_ON_BOOT, ARDUINO_USB_MODE) are removed from three ESP32 PlatformIO configuration files (esp32c3, esp32c6, esp32p4). Each file gains inline comments warning that these flags must not be re-added because they route Serial to USB-CDC, which may hang if no USB host is connected.

Changes

USB CDC Flag Removal

Layer / File(s)	Summary
Remove USB CDC flags and add warning comments platforms/platformio.esp32c3.ini, platforms/platformio.esp32c6.ini, platforms/platformio.esp32p4.ini	Removes -DARDUINO_USB_CDC_ON_BOOT and -DARDUINO_USB_MODE defines from build_flags in all three environments and inserts comments explaining that enabling USB CDC can cause Serial to hang at boot when no USB host is present.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐇 Three little configs, once causing a freeze,
No more USB CDC to hang on a breeze!
The UART stays cozy, the Serial is fine,
A wall adapter powers with no Serial whine.
Hop hop hooray, no more boot-time despair! 🎉

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/remove-toxic-usb-cdc

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