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/CHANGELOG.md

@@ -1,5 +1,16 @@
 # Changelog
 
+## 0.2.1
+
+`target_grid_w` (default -10 W): what the meter should rest at. The deadband
+holds any resting point inside it indefinitely, and the meter bills import and
+export on separate registers, so resting at +14 W import costs 0.34 kWh/day
+with the loop behaving perfectly. Biasing the target slightly negative moves
+that residue onto the export register. Configurable in the Configuration tab;
+see DOCS.md for the trade-off table.
+
+Behaviour is unchanged at `target_grid_w: 0`.
+
 ## 0.2.0
 
 Precedence between strategies is now a first-class object instead of an if/else