Initial release: ESP32-C3 CO2 & environment sensor board

KiCad 10 design (schematic, 2-layer layout, custom symbols and footprints),
fabrication data for PCBWay (Gerber X2, Excellon, job file), BOM, generated
documentation (schematic PDF, layer plots, 3D render) and firmware notes.

firmware/README.md

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+# Firmware
+
+ESPHome configuration for the ESP32-C3 CO₂ & environment sensor board.
+
+## Target
+
+| Item | Value |
+|---|---|
+| Chip | ESP32-C3 (RISC-V, single core, Wi-Fi + BLE) |
+| Framework | ESPHome (`esp32` platform, `framework: esp-idf` or `arduino`) |
+| Board | this project's board, flashed via the module's UART/USB boot pins |
+
+## Peripherals
+
+| Peripheral | Bus | Notes |
+|---|---|---|
+| Sensirion S8 (NDIR CO₂) | UART, 9600 baud 8N1 | needs 5 V supply, ~30 s preheat; `bCal_in` starts a manual background calibration |
+| BME280 (T / RH / P) | I²C, address `0x76` (some breakouts `0x77`) | powered from the module's 3.3 V output; pull-ups come from the breakout |
+| SW1 | GPIO, active low with 1 kΩ / 100 nF RC debounce | calibration button |
+
+## Add here
+
+- `esp-sensair.yaml` — the ESPHome configuration actually flashed to the board
+- `secrets.yaml` — **do not commit** (Wi-Fi credentials, Home Assistant token); keep
+  `secrets.yaml.example` with placeholders
+- flashing notes: `esptool` / `esphome flash`, boot-mode entry on the module
+
+## Home Assistant
+
+The device is exposed as native ESPHome entities: `co2`, `temperature`, `relative_humidity`,
+`pressure`, plus the calibration button as a sensor event.