Add ESPHome firmware templates, GND pour notes, v2 lessons learned

- firmware/: config + secrets templates committed, flashed config git-ignored
- README: document the GND pour on F.Cu/B.Cu and the module keep-out rule areas
- README: correct the calibration button wiring (S8 bCal_in, not an MCU GPIO)
- README: v2 list - dedicated 5 V power input, decoupling, mounting holes,
  I2C pull-ups, schematic annotation, RoHS finish, project name, antenna keep-out

firmware/esp-sensair.yaml.example

@@ -0,0 +1,72 @@
+# ESPHome configuration for the ESP32-C3 CO2 & environment sensor board.
+#
+# This is the SHARED TEMPLATE. Copy it to esp-sensair.yaml (git-ignored) and keep
+# all credentials in secrets.yaml (git-ignored). Never commit real keys.
+#
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2026 Hannes Gehrmann
+
+esphome:
+  name: esp32c3-bme280
+  friendly_name: esp32c3-bme280
+
+esp32:
+  board: esp32-c3-devkitm-1
+  framework:
+    type: esp-idf
+
+logger:
+
+api:
+  encryption:
+    key: !secret api_encryption_key
+
+ota:
+  - platform: esphome
+    password: !secret ota_password
+
+wifi:
+  ssid: !secret wifi_ssid
+  password: !secret wifi_password
+  fast_connect: true
+
+  manual_ip:
+    static_ip: !secret static_ip
+    gateway: !secret gateway
+    subnet: !secret subnet
+    dns1: !secret dns1
+
+  # Fallback hotspot if the Wi-Fi connection fails
+  ap:
+    ssid: "Esp32C3-Bme280 Fallback Hotspot"
+    password: !secret ap_password
+
+captive_portal:
+
+# BME280 on I2C - SDA on GPIO7, SCL on GPIO8 (board nets /SDA, /SCL)
+i2c:
+  sda: GPIO7
+  scl: GPIO8
+  scan: true
+
+# Sensirion S8 on UART - 9600 baud, 8N1 (board nets /TX, /RX)
+uart:
+  tx_pin: GPIO21   # ESP32-C3 TX -> S8 RX
+  rx_pin: GPIO20   # ESP32-C3 RX <- S8 TX
+  baud_rate: 9600
+
+sensor:
+  - platform: bme280_i2c
+    address: 0x76
+    temperature:
+      name: "BME280 Temperature"
+    pressure:
+      name: "BME280 Pressure"
+    humidity:
+      name: "BME280 Humidity"
+    update_interval: 60s
+
+  - platform: senseair
+    co2:
+      name: "Senseair S8 CO2"
+    update_interval: 60s