Fix GenIso.fields compilation after IsoFields wildcard change

[mbovel]

Summary

CI on master is failing since #1574 with:

[E007] Type Mismatch Error: macro/src/main/scala-3/monocle/macros/GenIso.scala:77:49
77 |        whitebox('{ monocle.internal.IsoFields[S](using $e) })
   |          Found:    monocle.PIso[S, S, ? <: Tuple, ? <: Tuple]
   |          Required: monocle.Iso[S, A]

#1574 changed IsoFields.apply to return PIso[S, S, ? <: Tuple, ? <: Tuple], but the fallback arm of GenIso._fields passes that expression to whitebox[A](e: Expr[Iso[S, A]]), and the wildcard type cannot unify with Iso[S, A] (which requires both type parameters to be the same A).

Fix

Widen whitebox to accept Expr[PIso[S, S, ?, ?]]. All three match arms conform to it (Expr is covariant and Iso[S, A] = PIso[S, S, A, A]), and the cast it performs is unchanged. Since GenIso.fields is transparent inline, the precise type is still inferred at call sites.

Verification

• sbt '++ 3' macrosJVM/test: compiles, 130/130 tests pass, including GenIso.fields on a two-field case class which exercises the fixed fallback arm.
• sbt '++ 3' macrosJVM/scalafmtCheck: passes.

🤖 Generated with Claude Code

[mbovel]

This fix is the minimal change; but I think we can do better and instead completely remove whitebox usage, like in #1574. Here as well, whitebox lies: Iso[S, Unit] is not a subtype of Iso[S, Any]. I am working on it.

[mbovel]

Closing in favor of #1602.