UX-54 round 6: the floor cut in one step, never from an empty battery

daycurve._floor_cut() set a cut from a battery already at the floor (its
cap 0 is below its rating), so a give run with one battery empty and the
other held to its rating was cut until MAX_FLOOR_CUTS ran out: 2,800 cuts
and 460 ms a build, and give blocks back at the hour edge after that.

The shortfall in a step is now shared as control shares it (by what each
battery holds above the floor, packshare.weights(), each at most its
rating, the rest passed on), in the step and in the cut. A battery giving
v kW that holds h kWh is empty at h / v; one at the floor holds nothing and
never sets a cut. The cut lands where the step then comes out exact, so it
is placed once. curve(stats=) reports the floor cuts and whether the bound
stopped one.

test_ux54.py: _floor_cut() checked to the second against floor times worked
out by hand (one battery, a group with one held to its rating, the second
group, the split by what they hold), a battery at the floor sets no cut,
the bound is reported; own_walk() with the reviewer's two cases (30/80 %,
guard, 7 kW house; 2 x 3 kW, 3.5 kW house) and, per build, the floor cuts,
the level points and the time.

No decision, command, timing or option key changes.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bdqr7PiaHcBnzPituu2yTC

goodwe_controller/DECISIONS/UX-54-forecast-tab.md

@@ -48,9 +48,14 @@ battery's expected level with its blocks. It follows the add-on, as Home does
   levels (`_mode_roles()` each quarter; the house from the profile, the sun
   at 100 % trust, a GIVE role counted only where control's own discharge
   envelope `packshare.envelope()` is open, the cut-out at the floor), charges
-  and gives in exactly the quarters the tab has a block in 11 cases
+  and gives in exactly the quarters the tab has a block in 13 cases
   (`test_ux54.py` part 7, `own_walk()`), e.g. 25/45 % with the peak guard:
-  control gives 17:00-20:15, the block is 17:00-20:06. With a cut-out under
+  control gives 17:00-20:15, the block is 17:00-20:06; 30/80 % with a 7 kW
+  evening house, B held to its rating: control 17:00-19:45, the block
+  17:00-19:32. The floor point is placed in one cut, where the battery runs
+  out at the rate it gives (`_floor_cut()`, checked to the second against
+  times worked out by hand), so a build stays small: at most 14 floor cuts
+  and 28 level points per battery and day in those cases. With a cut-out under
   the floor, or a house or sun that differs from the forecast and profile,
   control differs (Limits).
 - **The sun** is what `solarfc` reads. An hourly split (Solcast
@@ -64,7 +69,9 @@ battery's expected level with its blocks. It follows the add-on, as Home does
   `daycurve.curve()` derives it: each battery has the role `modetrade.decide()`
   gives (follow the house, hold, give, charge from the grid, rest), spare solar
   goes to Solar first batteries first, a shortfall is met by those that may
-  give, both at `dayplan.EFF`, inside the plan's floor and ceiling, from the
+  give, shared as control shares it (by what each holds above the floor,
+  `packshare.weights()`, each at most its rating, the rest passed on), both
+  at `dayplan.EFF`, inside the plan's floor and ceiling, from the
   level now. Spare solar is `trust x max(0, sun - house)` and what is stored
   loses `EFF`, the plan's own `trust x EFF x max(0, sun - house)` (10 kWh of
   sun, 2 kWh of house, 70 %: 5.32 kWh stored). A shortfall is not scaled by