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/control.py

@@ -28,6 +28,14 @@ class Tuning:
     step_w: int = 10
     saturation_w: float = 500.0
     saturation_cycles: int = 3
+    # What the meter should rest at, in W. Negative = a slight export.
+    # ⚠️ The deadband is a one-way ratchet: any resting point inside it holds
+    # forever, and the meter's IMPORT register counts every positive one with
+    # no export to cancel it. Resting at 0 W therefore leaks ~deadband/2 W of
+    # billed import all day (15 W deadband ≈ 0.2-0.35 kWh). Biasing the rest
+    # point below zero moves that leak into the export register, which is not
+    # billed. Cost is ~|bias| W of given-away export; keep it small.
+    target_grid_w: float = 0.0
 
 
 @dataclass(frozen=True)
@@ -74,11 +82,12 @@ def compute(
     # recognisable resting state, which is worth more than it looks - "flat for
     # 70 s" is how a healthy loop is recognised at a glance, and a command
     # frozen where it should not be is how two real bugs were caught.
-    if abs(grid_w) < tuning.deadband_w:
+    error = grid_w - tuning.target_grid_w
+    if abs(error) < tuning.deadband_w:
         want = prev_w
         reason = "deadband"
     else:
-        want = prev_w + tuning.gain * grid_w
+        want = prev_w + tuning.gain * error
 
     # ⚠️ Maintenance shaping (charge-only, cheap-window floor) used to live
     # here. It now belongs to arbiter.py as limit claims, so that precedence