docs(test): make Given/When/Then the default BDD structure (WP-71)

bdd.mdx previously banned "Given/When/Then ceremony" outright, which
directly contradicted WP-70's own acceptance tests (Acceptance/
BesluitLifecycleTests.cs already used // Given/When/Then comments) and
the backend's organically-evolved PascalCase_snake_sentence convention,
which the doc gave zero guidance for. Reverses that rule: every test is
now structured Given -> When -> Then, with a genuinely empty phase
omitted rather than faked; present-tense declarative naming and the
one-behaviour-per-test rule are unchanged. ADR-0006 gets a cross-reference
so both documents agree everywhere, not just in acceptance tests.

Also closes out the doc's other named-but-unenforced rules found by the
audit: fixes the 5 files asserting rendered $localize copy instead of
the underlying tag/message-id (the compliant pattern already existed in
werkvoorraad-item-view.spec.ts), splits the multi-behaviour titles the
doc itself calls a smell (";", "and", "/"), and fixes bdd.mdx's own false
citation of registratie-wizard.machine.spec.ts as "one transition per
test" by actually splitting that test into one-transition-per-test.

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

docs/reference/architecture/0006-test-data-builders.md

@@ -133,6 +133,17 @@ boundary row is the deliberate exception, not a contradiction: there the entire
 test is to exercise what happens when the input _isn't_ valid, so the fixture must be able to
 represent the invalid shape a builder would refuse to construct.
 
+## A note on Given/When/Then and `bdd.mdx`
+
+This ADR's "Given.Concept()...Build()" builder chain and the acceptance-test row above
+("composed into one Given→When→Then read") already used Given/When/Then before it was the
+repo-wide default. `libs/shared/docs/bdd.mdx` has since made G/W/T structure — a `// Given` /
+`// When` / `// Then` comment (or, in TypeScript, the equivalent unlabelled ordering) inside
+every test body, acceptance or not — the documented convention for **all** tests, not only
+acceptance ones (reversing its own earlier "no G/W/T ceremony" rule). The two documents now
+agree everywhere: this ADR's `Given` builder is the fixture idiom; `bdd.mdx` rule 1 is the
+structural convention every test using that fixture (and every other test besides) follows.
+
 ## Consequences
 
 - **+** An illegal backend fixture (e.g. a decided-but-not-submitted `Aanvraag`) is now a