refactor: one member order for the 3 wizard containers (RD-38)

RD-22 and RD-23 brought the wizard containers under the 250-line budget, so
`max-lines` reports nothing. The files still read badly. Line count was never
the problem.

Fix three things in all three containers:

1. The member order was scrambled, and it differed per file. `registratie`
   declared `draftSync` in the middle of a run of `computed`s; `herregistratie`
   read `this.stepLabels.length` seven lines before `stepLabels` existed; the
   three files put the copy arrays in three different places. All three now use
   one nine-section order, so they compare side by side.
2. Pure logic sat in the container. Extract `digitalDocumentIds` into
   `upload.machine.ts` — the "digital and finished uploading" filter was
   written out four times, and it removes a `documentId!` assertion from both
   containers. Extract `diplomaMsg` into a sibling of the step files.
3. Comments carried archaeology. Drop the three RD-05 references and keep the
   rule. Drop "replaces sessionStorage" and the note about focus management that
   moved to the shell. Fix `intake`'s class comment, which claimed answers
   persist to sessionStorage and was contradicted 30 lines below.

`phase` deliberately stays in all three: it cannot live in `domain/`, and three
siblings plus three specs is a worse trade than 17 readable lines. The store ⇄
`draftSync` cycle also stays — both callbacks are deferred, so it is safe, and
one comment now names it.

No behaviour change. Member lists and every `private`/`protected`/`readonly`
modifier are unchanged, which the showcase depends on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 556 frontend behaviours across
+**is** the suite, reshaped for a business reader. 562 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -545,6 +545,12 @@ classes.
 - lists soort/waarvoor/status/referentie/ingediend, plus reason when rejected
 - reference falls back to em dash for a Concept
 
+#### diplomaMsg
+
+- resolves a known diploma into KiesDiploma with the server-derived beroep
+- resolves the manual sentinel into KiesHandmatig with the maximal question set
+- returns null for an unknown diploma id
+
 #### findConcept
 
 - returns the id of the existing Concept of the given type
@@ -857,6 +863,12 @@ classes.
 - emits documentId for completed digital uploads and channel for post
 - omits digital categories with no completed upload
 
+#### digitalDocumentIds
+
+- returns the id of a completed digital upload
+- omits an upload that is still in flight
+- omits a category that the user delivers by post
+
 #### flushPendingGuard
 
 - flushes then allows navigation when a write is pending