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/DOCS.md

@@ -56,6 +56,7 @@ phase having charged nothing.
 | `gain` | 0.6 | Correction per cycle. **At the limit — do not raise** |
 | `slew_w` | 1000 | Maximum change per cycle |
 | `deadband_w` | 15 | Ignore errors smaller than this |
+| `target_grid_w` | -10 | What the meter should rest at. Negative = a slight export |
 | `step_w` | 10 | Quantisation |
 | `saturation_w` | 500 | Divergence that counts as "the inverter is at a limit" |
 | `saturation_cycles` | 3 | How many consecutive cycles before freezing. **Do not set to 1** |
@@ -63,6 +64,30 @@ phase having charged nothing.
 | `stale_input_s` | 15 | How long inputs may be missing before commanding 0 W |
 | `auto_start` | false | Start controlling on boot (only after commissioning) |
 
+#### Why `target_grid_w` is not zero
+
+The deadband is a one-way ratchet: any resting point inside it holds until
+something disturbs it. Import and export are **separate registers on the
+meter**, so a rest point of +14 W is billed for every second it holds and no
+amount of export cancels it - 14 W all day is 0.34 kWh.
+
+Biasing the target below zero moves that residue into the export register,
+which is not billed. The resting band becomes `target ± deadband`, so:
+
+| `target_grid_w` | resting band | worst billed leak | export given away |
+|---|---|---|---|
+| 0 | -15 … +15 W | ~15 W (0.35 kWh/day) | none |
+| **-10** | -25 … +5 W | ~5 W (0.12 kWh/day) | ~10 W |
+| -15 | -30 … 0 W | none | ~15 W (0.36 kWh/day) |
+
+Set it to `-deadband_w` if injection is worth nothing to you and you would
+rather give the energy away than buy it back. Set it to `0` if you are paid
+properly for export, or if you are debugging and want the loop centred.
+
+⚠️ This is a **billing** knob, not a speed knob. If import is arriving in
+bursts rather than as a trickle, the cause is tracking lag, and this will not
+help - see "Why the tuning is what it is".
+
 ### Maintenance
 
 | option | default | meaning |