Initial ESP32-C3 single-nail firmware with Wi-Fi onboarding

README.md

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+# piNail ESP32-C3 (Single-Nail)
+
+ESP32-C3 firmware for a single-nail piNail controller.
+
+## What changed vs piNail2
+- Single nail only.
+- Loop timing is fixed for responsiveness (`1800ms` cycle, `200ms` tick).
+- Loop timing controls are intentionally not exposed in API/UI.
+- Wi-Fi onboarding uses captive portal (`piNail-Setup` / `pinailsetup`).
+
+## Hardware target
+- Board: ESP32-C3 DevKitM-1 class board.
+- Thermocouple ADC: MAX6675 (SPI-like bitbang).
+- Relay output: active HIGH relay module.
+
+## GPIO mapping
+- Relay: `GPIO2`
+- MAX6675 SCK: `GPIO4`
+- MAX6675 CS: `GPIO5`
+- MAX6675 SO: `GPIO6`
+
+## API compatibility
+Implemented endpoints:
+- `GET /api/status`
+- `GET /api/history`
+- `POST /api/power`
+- `POST /api/setpoint`
+- `POST /api/pid`
+- `POST /api/pid/reset`
+- `POST /api/safety/reset`
+- `GET /api/heartbeat`
+
+Not implemented in this firmware:
+- Scheduler and dual-nail routing.
+- Autotune endpoints.
+- Loop timing reconfiguration endpoint.
+
+## Build and flash
+```bash
+pio run -t upload --upload-port /dev/ttyACM0
+pio device monitor -b 115200 --port /dev/ttyACM0
+```