Fix merged polymorphic this reference analysis

[MatthewHarrigan]

Summary

• Fix runaway type instantiation for merged class/interface declarations where polymorphic this has multiple declarations.
• Select the declaration containing the node before checking for possible type-parameter references, while retaining the conservative fallback when it cannot be identified.
• Add regression coverage for merged polymorphic this and genuine recursive-reference cases.

Performance

On the minimized reproduction:

• Instantiations: 5,002,154 to 2,008
• Check time: approximately 25 seconds to approximately 0.004 seconds
• Peak RSS: multiple GB to approximately 61 MB

Validation

• npx hereby build
• npx hereby test
• npx hereby format
• npx hereby lint
• Full local and upstream compiler baseline suites

AI assistance

This patch was developed with assistance from OpenAI Codex. I have reviewed and understand the changes and will shepherd the review.

[Copilot]

Pull request overview

Fixes runaway type instantiation when analyzing polymorphic this in merged class/interface declarations while preserving conservative recursive-reference handling.

Changes:

• Selects the merged declaration containing the analyzed node.
• Adds regression tests for merged this and finite recursive nesting.
• Adds expected diagnostic baselines for recursive incompatibilities.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/checker.go	Refines type-parameter reference analysis.
testdata/tests/cases/compiler/mergedPolymorphicThisReferences.ts	Tests merged polymorphic this.
testdata/tests/cases/compiler/selfReferentialInstanceMemberRecursion.ts	Covers the performance regression.
testdata/tests/cases/compiler/selfReferencePreservesFiniteNesting.ts	Tests recursive nesting.
testdata/tests/cases/compiler/selfReferenceInMethodBody.ts	Tests references inside methods.
testdata/tests/cases/compiler/selfReferenceExactInstancesPreserveFiniteNesting.ts	Tests exact recursive instances.
testdata/tests/cases/compiler/typePredicateRecursionPreservesFiniteNesting.ts	Tests recursive type predicates.
testdata/baselines/reference/compiler/selfReferencePreservesFiniteNesting.errors.txt	Records expected diagnostics.
testdata/baselines/reference/compiler/selfReferenceInMethodBody.errors.txt	Records expected diagnostics.
testdata/baselines/reference/compiler/selfReferenceExactInstancesPreserveFiniteNesting.errors.txt	Records expected diagnostics.
testdata/baselines/reference/compiler/typePredicateRecursionPreservesFiniteNesting.errors.txt	Records expected diagnostics.

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