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/secrets.yaml.example

@@ -0,0 +1,14 @@
+# Copy to firmware/secrets.yaml (git-ignored) and fill in your own values.
+#
+# Generate a Home Assistant API encryption key with:
+#   python3 -c "import base64,os; print(base64.b64encode(os.urandom(32)).decode())"
+
+wifi_ssid: YourNetworkSSID
+wifi_password: YourNetworkPassword
+api_encryption_key: REPLACE_WITH_GENERATED_32_BYTE_BASE64_KEY
+ota_password: REPLACE_WITH_AN_OTA_PASSWORD
+ap_password: REPLACE_WITH_A_FALLBACK_HOTSPOT_PASSWORD
+static_ip: 192.168.1.50
+gateway: 192.168.1.1
+subnet: 255.255.255.0
+dns1: 192.168.1.1