SAFETY-04: clamp the integrator, not only the output

The loop was in velocity form: the accumulator WAS the commanded power, so
"clamp the integrator" and "clamp the output" were the same line of code and
could not be set apart. This makes the accumulator an explicit carried value
(`i_w`), bounds it with its own `integrator_max_w`, and keeps the output clamp
where it was. Killing either mechanism now still leaves the other holding -
which is the point of the ticket, and what the new regression test asserts.

Freeze semantics: while saturated the integrator may unwind but not wind
further. A strict freeze would strand the command at whatever it reached,
because the condition that releases it is the inverter tracking again, and not
tracking is exactly what saturation means.

The integrator is re-seeded from the arbiter's actual output whenever the loop
did not get what it asked for, so entering any failsafe (all of which resolve
to 0 W) zeroes it, and the first cycle after release does not dump the stale
period as power.

test_control.py: 24 -> 33 checks, all passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Du77usMj8XNKNFZGmUiWDa

goodwe_controller/DOCS.md

@@ -60,6 +60,7 @@ phase having charged nothing.
 | `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** |
+| `integrator_max_w` | 3000 | Bound on the loop's accumulator, separate from `max_w`. Caps how much stale error can be waiting to unwind when the sign flips. **Keep it above `max_w`, and do not set it equal to `max_w`** |
 | `heartbeat_s` | 10 | Refresh interval; must stay well under the firmware watchdog |
 | `stale_input_s` | 15 | How long inputs may be missing before commanding 0 W |
 | `auto_start` | false | Start controlling on boot (only after commissioning) |