Fix E1018 false positive on Fn::Split over Fn::GetStackOutput

[satyakigh]

Summary

Fn::Split whose source is Fn::GetStackOutput was reported as E1018 (Fn::Split source must be a string or a string-producing intrinsic), but CloudFormation accepts that nesting. The AWS CDK synthesizes exactly this shape for weak string-list cross-stack references (Fn::Split("||", Fn::GetStackOutput ...)), so every such consumer stack tripped the rule, and the CDK's CloudFormationValidatePlugin currently suppresses E1018 outright to work around it.

The allowed Fn::Split sources mirrored the published Fn::Split operand list, which predates Fn::GetStackOutput and still omits it. This change adds Fn::GetStackOutput to SPLIT_SOURCE_FUNCTIONS in template-model, the shared layer both engines use, so the fix applies to Rego and CEL identically.

• src/template-model/src/intrinsic_arg_shapes.rs: add FN_GET_STACK_OUTPUT to SPLIT_SOURCE_FUNCTIONS; new unit test split_source_get_stack_output_is_allowed.
• src/resources/templates/good/functions/split_get_stack_output.yaml: good-corpus fixture in the deploy-verified shape (split used as a list property, re-joined into a string with Fn::Join, and indexed with Fn::Select).
• src/resources/expected/validation_reports{6,7,8}.json: regenerated; the only new entries are the fixture's three I9040 (INFO, missing Tags) findings, the rest is chunk-boundary movement. No existing template's diagnostics changed.

With this fix released, the E1018 entry in the CDK plugin's IGNORE_RULES becomes unnecessary.

• Documentation: the Fn::Split reference lists the same ten source functions as the previous allowlist and predates Fn::GetStackOutput. The Fn::GetStackOutput reference enumerates supported positions (direct property value, Fn::Join, Fn::If, Fn::Select) and known limitations (Fn::Sub variable map, Fn::Base64, Outputs, Fn::Equals, Fn::ImportValue); Fn::Split appears in neither list, and the deployment shows it is accepted.
• CDK: aws-cdk-lib emits this nesting for weak string-list cross-stack references (@aws-cdk/core:defaultCrossStackReferences=weak), and its CloudFormationValidatePlugin lists E1018 in IGNORE_RULES for that reason.

Checklist

• For validation behavior changes, expected behavior is supported by CloudFormation evidence and all three engine
  selectors agree.
• My code adheres to the [CONTRIBUTING GUIDE][contributing-guide] and DESIGN GUIDELINES.

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