Fix export default dts widening

[jakebailey]

Two things:

• For export assignments, prevent widening when serializing the symbol. This used to be done with preserveLiterals.
• The "big shortcut" (which I'm pretty sure I suggested) breaks these literal cases, because we ask the checker to widen it.

[Copilot]

Pull request overview

This PR fixes incorrect widening of literal types in emitted .d.ts for export default/export assignments, aligning tsgo’s declaration output and incremental behavior more closely with TypeScript’s (and removing previously-recorded expected diffs).

Changes:

• Preserve literal types when serializing types for export assignments (avoids widening 1/"string"/42 to number/string).
• Adjust pseudotype literal handling to avoid forcing widening via the “big shortcut”.
• Update/clean up reference baselines and incremental test expectations now that diagnostics and signatures are correct.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/checker/nodebuilderimpl.go	Avoids widening literal types when serializing types for export assignments.
internal/checker/pseudotypenodebuilder.go	Stops widening for pseudotype literal mapping to prevent incorrect literal loss.
internal/execute/tsctests/tsc_test.go	Removes prior expectedDiff now that the underlying issue is fixed.
testdata/baselines/reference/tsc/incremental/change-to-type-that-gets-used-as-global-through-export-in-another-file.js	Baseline update: default export retains literal type; incremental diagnostics/signatures updated.
testdata/baselines/reference/tsc/incremental/change-to-type-that-gets-used-as-global-through-export-in-another-file-through-indirect-import.js	Baseline update: same as above, via re-export chain.
testdata/baselines/reference/tsc/composite/synthetic-jsx-import-of-ESM-module-from-CJS-module-no-crash-no-jsx-element.js	Baseline update: default export literal type preserved (42).
testdata/baselines/reference/submodule/conformance/jsDeclarationsDefault(target=es2015).js	Baseline update: default export literal type preserved (12).
testdata/baselines/reference/submodule/conformance/jsDeclarationsDefault(target=es2015).js.diff	Baseline diff updated to reflect convergence.
testdata/baselines/reference/submodule/compiler/nodeNextCjsNamespaceImportDefault2.js	Baseline update: default export string literal preserved ("string").
testdata/baselines/reference/submodule/compiler/nodeNextCjsNamespaceImportDefault2.js.diff	Baseline diff removed/updated due to convergence.
testdata/baselines/reference/submodule/compiler/modulePreserve4.js	Baseline update: preserves 0 literals for relevant default exports.
testdata/baselines/reference/submodule/compiler/modulePreserve4.js.diff	Baseline diff updated to reflect convergence.
testdata/baselines/reference/submodule/compiler/dynamicImportsDeclaration.js	Baseline update: preserves literal defaults (0, 1, "fallback").
testdata/baselines/reference/submodule/compiler/dynamicImportsDeclaration.js.diff	Baseline diff removed/updated due to convergence.
testdata/baselines/reference/compiler/jsdocCommentDefaultExport(module=esnext).js	Baseline update: preserves literal type in default export with JSDoc.
testdata/baselines/reference/compiler/jsdocCommentDefaultExport(module=commonjs).js	Baseline update: preserves literal type in default export with JSDoc.