Preserve inline parameter comments on inferred function types in declaration emit

[Copilot]

tsc retains inline trailing comments on arrow-function/function-expression parameters in .d.ts output (stripping only the last parameter's, as a side effect of list emit), but tsgo dropped them entirely for inferred types.

export const fn = (
    foo: boolean, // comment on foo
    bar: string, // comment on bar
    buzz: number,  // comment on buzz
) => {};

// before
export declare const fn: (foo: boolean, bar: string, buzz: number) => void;

// after (matches tsc)
export declare const fn: (foo: boolean, // comment on foo
bar: string, // comment on bar
buzz: number) => void;

Root cause

Declaration emit for inferred types uses the pseudochecker (port of TS's expressionToTypeNode). For an arrow with an un-inferrable return (e.g. empty body => {}), typeFromFunctionLikeExpression short-circuited to NewPseudoTypeInferred(node), discarding the syntactically-built parameters. They were then rebuilt fully synthesized by the checker with no source range, so the printer had no positions from which to emit trailing comments.

Changes

• internal/pseudochecker/lookup.go — Drop the NoResult short-circuit so the function is always built as a SingleCallSignature (matching TS), preserving parameter source positions; the return type is still serialized from the checker's inferred type.
• internal/checker/pseudotypenodebuilder.go
  • pseudoParameterToNode: copy the original parameter declaration's range onto the synthesized node (guarded to the enclosing file) so the printer emits trailing comments.
  • pseudoTypeEquivalentToType (single-call-signature case): treat a NoResult return as equivalent (checker supplies it), and strip optional | undefined from the target before resolving its call signature so nested function-expression parameter defaults don't emit spurious TS9013.
• Test: add declarationEmitArrowParameterComments.ts.

Baseline impact

• isolatedDeclarationErrorsReturnTypes: 59 → 31 errors, now exactly matching TS (triaged diff removed).
• commentsFunction(es2015): now preserves JSDoc parameter comments (accepted diff removed).
• isolatedDeclarationErrors: error count now matches TS.