fix(envea): correct serial read to prevent spurious spikes

- Replace readline() with read(32) to avoid truncation at 0x0A bytes
- Add reset_input_buffer() to clear stale data before each read
- Add initial read to consume echo/acknowledgment from sensor
- Add frame header validation (0xFF 0x02) to reject invalid data
- Add delays to allow sensor response time

Fixes issue where NO2/H2S sensors showed random spikes due to
binary data containing newline characters being misinterpreted.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

envea/read_value_v2.py

@@ -106,16 +106,31 @@ try:
                 try:
                     debug_print(f"Reading from {name}...")
 
+                    # Flush input buffer to clear any stale data
+                    serial_connection.reset_input_buffer()
+
                     # Send command to sensor
                     command = b"\xFF\x02\x13\x30\x01\x02\x03\x04\x05\x06\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\x12\xAF\x88\x03"
                     serial_connection.write(command)
                     debug_print(f"  → Sent command: {command.hex()}")
 
-                    # Read response
-                    data_envea = serial_connection.readline()
-                    debug_print(f"  ← Received {len(data_envea)} bytes: {data_envea.hex()}")
+                    # Wait for sensor response
+                    time.sleep(0.5)
 
+                    # Read fixed number of bytes (not readline which stops at 0x0A)
+                    # First read to consume any echo/acknowledgment
+                    initial_data = serial_connection.read(serial_connection.in_waiting or 1)
+                    debug_print(f"  ← Initial read: {len(initial_data)} bytes")
+
+                    # Small delay then read the actual response
+                    time.sleep(0.1)
+                    data_envea = serial_connection.read(32)  # Read fixed 32 bytes
+                    debug_print(f"  ← Received {len(data_envea)} bytes: {data_envea.hex() if data_envea else 'empty'}")
+                    
+                    # Validate frame: should start with 0xFF 0x02 and have at least 20 bytes
                     if len(data_envea) >= 20:
+                        # Check frame header (0xFF 0x02 expected for valid CAIRSENS response)
+                        if data_envea[0] == 0xFF and data_envea[1] == 0x02:
                             byte_20 = data_envea[19]
                             raw_value = byte_20
                             calculated_value = byte_20 * coefficient
@@ -137,7 +152,9 @@ try:
 
                             debug_print(f"  ✓ {name.upper()} = {calculated_value}")
                         else:
-                        debug_print(f"  ✗ Response too short (expected ≥20 bytes)")
+                            debug_print(f"  ✗ Invalid frame header: expected FF 02, got {data_envea[0]:02X} {data_envea[1]:02X}")
+                    else:
+                        debug_print(f"  ✗ Response too short ({len(data_envea)} bytes, expected ≥20)")
                         
                 except serial.SerialException as e:
                     debug_print(f"✗ Error communicating with {name}: {e}")