SAFETY-137: the shutdown and Stop 0 W is the last write every unit hears

The 0 W of shutdown and of the Stop button now waits (bounded, 2 s) for a
write already on its way to the same unit, for every driver, GoodWe included.
Once shutdown has begun, cycle, heartbeat and safety-stop writes are dropped
right before the write is created. One shared wait (_await_inflight) serves
the 0 W and SMA-10's _release_last.

Co-Authored-By: Claude <noreply@anthropic.com>

goodwe_controller/DECISIONS/SAFETY-137-last-write-is-zero.md

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+# SAFETY-137 - the shutdown 0 W is the last write every unit hears
+
+SMA-10 made a hand-back release wait for a write already on its way
+(`_release_last`, `Inverter.inflight`). A GoodWe unit skipped that, and rig 5b
+saw a cycle write (-2 823 W) land after the shutdown 0 W and stay until relaunch.
+
+Now the 0 W of shutdown and of the Stop button is a *last* write
+(`_write_unit(last=True)`), for every driver:
+
+- It first waits, at most `SETTLE_BUDGET_S` (2 s), for the writes already on
+  its way to that unit (`_await_inflight`, shared with `_release_last`). A hung
+  write is not waited out and not cancelled; the 0 W goes anyway.
+- Once shutdown has begun (`stopping`), every other write (cycle, heartbeat,
+  safety stop) is dropped at the same place, right before the write is created.
+  That is the gate that keeps the control cycle quiet; the cycle itself needs
+  no cancelling.
+- Worst case for a GoodWe unit is 2 s + `WRITE_BUDGET_S` (4 s) = 6 s, the same
+  as `HAND_BACK_WORST_S`; the units are handled together.
+
+Not touched: the heartbeat's own timing, safety-stop paths, the maintenance
+abort (still a plain 0 W).