laundry: CloudExtraCourse_ means two different things; tell them apart

Its bytes are not payload slots everywhere. On the DW5000C dishwasher all
four (8E 8D 8F 02) are course codes in that device's own course list, three
already translated -- Plastic, Pots and pans, Baby Care. There the token
marks which ordinary courses came from the cloud; they select with a plain
Course_ write and need no payload, which is consistent with it carrying no
payload token at all. It also has a DownloadCourseList_ token the washers
lack. On both washers the slots share zero overlap with the course list and
a payload is required to select one.

So the "this is not washer-only" claim was wrong, and gating on
advertised_slots offered that dishwasher's owner a naming flow for programs
that already work and are already named. The Repairs card was spared only
because the payload gate added earlier happens to catch it.

cloud_slots() subtracts the device's own course list, which separates the two
readings without guessing at families: what remains is slots that cannot be
selected any other way, which is what this module is for. Everything
user-facing now gates on that -- the options menu entry, the naming flow, the
Repairs count. The dishwasher gets nothing, both washers are unchanged.

Found by reading the dishwasher fixture's options array while answering a
question about it, which is also why diagnostics now reports advertised and
cloud slots separately: the difference between them is the whole distinction.

docs/investigations/download-cycle.md

@@ -15,16 +15,25 @@ no cloud tokens at all, so this is a minority feature):
 | --- | --- | --- | --- | --- |
 | `washer_ww5000c_cloud` | WW5000C `_B06C`, `DA_WM_TP1_21_COMMON`, Table_02 | 9 | 2 | 20 bytes |
 | `washer_wa55a7700av` | WA55A7700AV, `DA_WM_TP1_21_COMMON`, Table_02 | 2 | 1 | 16 bytes |
-| `dishwasher_dw5000c_cloud` | DW5000C, `DA_DW_TP1_21_COMMON` | 4 | **0** | — |
+| `dishwasher_dw5000c_cloud` | DW5000C, `DA_DW_TP1_21_COMMON` | 4 (but see below) | **0** | — |
 | (not fixtured) | WW5000C `_B048`, issues #259/#343, Table_02 | 9 | 1 | 20 bytes |
 
 Three things follow immediately from that table:
 
-- **It isn't washer-only.** The DW5000C is a `DA_DW_` dishwasher.
-- **A device can advertise programs it has never loaded.** The DW5000C names
-  four slots and carries no `CloudCourse_`/`OneTimeCloudCourse_` token at
-  all. Nothing about it is learnable until its owner runs one, which is
-  exactly the situation the Repairs issue exists to explain.
+- **`CloudExtraCourse_` does not mean the same thing on every family.** On
+  the DW5000C all four of its bytes (`8E 8D 8F 02`) are course codes in that
+  dishwasher's *own* course list, three already translated (Plastic, Pots and
+  pans, Baby Care). There it tags which ordinary courses came from the cloud;
+  they select with a plain `Course_` write and need no payload — consistent
+  with it carrying no payload token at all. It also has a
+  `DownloadCourseList_8F` token the washers lack.
+  On both washers the slots share **zero** overlap with the course list and a
+  payload is required. Subtracting the course list is what tells the two
+  apart (`cloudcourse.cloud_slots`), so the feature engages on the washers
+  and correctly does nothing on the dishwasher.
+- **A device can advertise a slot it has never loaded.** True on the washers
+  too — nothing about a program is learnable until its owner runs it, which
+  is what the Repairs issue exists to explain.
 - **Both WW5000C units advertise the byte-identical slot list**
   (`0A5C286B2D0C55301A`, same nine slots in the same order) despite different
   firmware builds. Either the set is a factory/regional default rather than