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/app/main.py

@@ -66,6 +66,7 @@ class Controller:
             max_w=float(opts.get("max_w", 2000)),
             slew_w=float(opts.get("slew_w", 1000)),
             deadband_w=float(opts.get("deadband_w", 15)),
+            target_grid_w=float(opts.get("target_grid_w", 0)),
             step_w=int(opts.get("step_w", 10)),
             saturation_w=float(opts.get("saturation_w", 500)),
             saturation_cycles=int(opts.get("saturation_cycles", 3)),