Fix functor module type result-signature brace formatting

[MavenRain]

Fix functor module type result-signature brace formatting

Fixes #7949.

Problem

When a functor module type returns a signature, the formatter broke the
opening brace onto its own line:

// input
module Make: Pattern => {
  let fmt: event => string
}

// formatted (before)
module Make: Pattern =>
{
  let fmt: event => string
}

This is inconsistent with functor module expressions, which already keep the
brace on the same line as =>:

module Make = (P: Pattern) => {
  let x = 1
}

Cause

In print_mod_type (the Pmty_functor arm of compiler/syntax/src/res_printer.ml),
the arrow was followed by a breakable Doc.line. Because a signature result
force-breaks internally, the enclosing group always broke at that Doc.line,
pushing { to the next line. The sibling print_mod_functor (module
expressions) uses an unconditional hard space (Doc.text " => ") and so never
had this problem.

Fix

When the functor result is a signature, use a hard space before it (hug the
brace); otherwise keep the breakable Doc.line so long non-signature results
(idents, with constraints, curried arrows) can still wrap:

let arrow_sep =
  match return_type.pmty_desc with
  | Pmty_signature _ -> Doc.space
  | _ -> Doc.line
in

Testing

• New printer test tests/syntax_tests/data/printer/modType/functorInterface.resi
  covering the reported .resi case plus curried params, a nested/curried arrow,
  an empty signature, and non-signature results (which must remain unchanged).
• Two existing golden outputs that had captured the wrong behavior are updated
  (modType/functor.res.txt module type B = () => {, and
  modExpr/structure.res.txt module G0: (X: {}) => {).
• make test-syntax and make test-syntax-roundtrip pass; the printer +
  idempotency corpus round-trips with no diff, and comments around => / inside
  the result signature are preserved.

_{Drafted with AI assistance; the change, tests, and verification were reviewed by me.}

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