Pin the DefaultRead-under-Outcome validate-time rejection with a named test

FetcherEmitter.resolvesViaPropertyDataFetcher answers true for two dispositions: an ErrorsField on the PayloadAccessor transport, and a ChildField.RecordReadField whose ValueLocator is DefaultRead. The first is pinned by FetcherPipelineTest wiring assertions (its own javadoc says so); the second is not pinned anywhere. The validator consumes the predicate in GraphitronSchemaValidator.validateOutcomeChildArmSwitch to reject a WrapperArm errors transport whose sibling data field would read a property off the Outcome source object instead of unwrapping Success, and OutcomeTypeValidationTest covers only the negative direction (outcomePayloadWithTableDataField_isNotRejected): no test asserts that the DefaultRead sibling actually rejects. Two claims rest on that unpinned chain. First, FetcherEmitter.inlineSuccessRead’s `DefaultRead arm throws and documents itself as unreachable because the validator rejects the combination first; nothing fails if the rejection stops firing, and the failure mode is a generator IllegalStateException instead of a located build error. Second, isInlineArmSwitchedDataField excludes DefaultRead for the same reason, also unpinned. Per "Every invariant has an enforcer" in docs/architecture/principles/development-principles.adoc, a claimed pin with no named test is the review-only label the axiom warns about. Reachability is worth settling as part of the item: DefaultRead arises for @error-type parents (which carry no WrapperArm errors field, so not that route) and for class-backed parents whose backing class did not load, so the live population is a payload result type with an unresolvable backing class. If the combination is genuinely unreachable, the honest fix is to pin that rather than the rejection, and to say so where the arm claims unreachability.

Filed from the R51 In Review → Done review; the extension of resolvesViaPropertyDataFetcher was an implementer addition beyond that item’s spec, whose acceptance bar deliberately scoped out per-locator-arm tests.