Retire the generated-body string-scan helpers in TypeSpecAssertions

TypeSpecAssertions exists to replace the assertThat(method.code().toString()).contains(...) pattern that docs/architecture/principles/development-principles.adoc bans, but four of its helpers implement that pattern internally: hasFieldsArm, appendsRequiredColumn, armGuardsArgumentConsistency, and serviceChildKeyExtractionIsUnconditional all scan a rendered method body for a substring. The file’s own javadoc argues this is acceptable because the fragility is confined to one place, which is a real improvement over scattering the scans across call sites, but it is a containment argument rather than a structural one: the assertions still break on emitter formatting changes that alter nothing observable, and a negative scan is worse still, since serviceChildKeyExtractionIsUnconditional asserts the absence of instanceof and so silently flips if any unrelated instanceof ever appears in that fetcher method.

The fix is to ask the structural question against the CodeBlock/MethodSpec tree instead of the rendered string, so the assertions name emitted structure rather than emitted text. Worth pricing against the fact that R549’s keystone slice retires the projection walk these helpers audit: if that lands first, some of the family goes away on its own and only the survivors need rehoming.

Raised by the independent Done-gate reviewer on the PK-only service key contract item, which net removed one member of the family and was explicitly not asked to fix the rest.

Scope extension from the facts-and-commands Done-gate review (2026-08-01): the renderer arm tests (ProjectionUnitRendererTest, RootLauncherRendererTest) open-code the same rendered-string scan (code().toString() + contains(...)) at their call sites, outside the TypeSpecAssertions containment this item was filed against, and armProjectsColumn / armGuardsArgumentConsistency joined the contained family as successors of the retired appendsRequiredColumn. The testing.adoc renderer-arm rubric row blesses "per-arm structural assertions" but is silent on assertion mechanics; when this item lands the structural CodeBlock/MethodSpec form, the renderer arm tests are in scope, and the rubric row should state the mechanics so the ambiguity closes with the migration.