Single-source the null-source-guarded data-channel predicate as a classifier-assigned fact

"This fetcher short-circuits on a null source" is a fact asserted twice with nothing binding the two sites: the emitter decides it structurally (the source-shape fork in TypeFetcherGenerator.buildBatchedDataFetcher emits the env.getSource() == null prelude on the Record arm and the instanceof Success narrowing on the Outcome arm, plus FetcherEmitter.buildSingleRecordIdFromReturningFetcherValue’s guard), while `GraphitronSchemaValidator.isLocalContextGuardedDataChannel re-enumerates the safe variants as a hand-maintained allow-list whose javadoc pleads "removing the guard from an existing emitter arm must remove the variant here". Per "Decide once, at the parse boundary" (two consumers evaluating the same predicate over model facts means the branch belongs in the model), lift the guarded-data-channel verdict into a classifier-assigned fact (an accessor on the field variant or a capability), have both the emitter prelude fork and the validator read it, and delete the allow-list switch. Also decide the currently-open edge the hand-list leaves ambiguous: BatchedLookupTableField routes through the same guarded builder arms but is not admitted by the validator; single-sourcing forces that verdict to be stated once, with validation coverage. Routed from the stale-comment corrections item that truth-verified the allow-list’s claims against the live emitter arms.