Rest the meter just below zero, not at zero

The deadband is a one-way ratchet: any resting point inside it holds
indefinitely. Import and export are separate registers on the meter, so a
loop resting at +14 W bills 0.34 kWh/day while behaving perfectly.

target_grid_w (default -10 W) moves that residue onto the export register.
Worst billed rest point drops from 15 W to under 5 W. Behaviour is unchanged
at target_grid_w: 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

goodwe_controller/config.yaml

@@ -1,5 +1,5 @@
 name: GoodWe RS485 Controller
-version: "0.2.0"
+version: "0.2.1"
 slug: goodwe_controller
 description: >-
   Drives a GoodWe ES/BP battery inverter over RS485 by emulating its smart
@@ -44,6 +44,7 @@ options:
   gain: 0.6
   slew_w: 1000
   deadband_w: 15
+  target_grid_w: -10
   step_w: 10
   saturation_w: 500
   saturation_cycles: 3
@@ -83,6 +84,7 @@ schema:
   gain: float(0.05,1.0)
   slew_w: int(50,5000)
   deadband_w: int(0,500)
+  target_grid_w: float(-200,200)
   step_w: int(1,100)
   saturation_w: int(100,2000)
   saturation_cycles: int(1,10)