Fix compatible-mode remap of pointer-to-fnptr-typedef fields

[tannergooding]

In compatible mode (ExcludeFnptrCodegen) a function-pointer typedef is emitted as a managed delegate. GetTypeNameForPointeeType short-circuited a remapped typedef to a pointer of the remapped name, so a field pointing at such a typedef emitted a pointer to the managed delegate (e.g. PFoo*), which is invalid -- it produces CS8500 and forces the containing struct unsafe.

Given:

typedef void (ApiPFoo)(int);
struct ApiCallbacks { ApiPFoo* Foo; };

with --remap ApiCallbacks=Callbacks ApiPFoo=PFoo in compatible mode.

Before:

public unsafe partial struct Callbacks
{
    [NativeTypeName("ApiPFoo *")]
    public PFoo* Foo;
}

After (matches the no-remap case; the delegate is still renamed to PFoo, only the field changes):

public partial struct Callbacks
{
    [NativeTypeName("ApiPFoo *")]
    public IntPtr Foo;
}

The fix mirrors the non-remapped path: in the wasRemapped branch, when ExcludeFnptrCodegen is set and the typedef's underlying type is a function type, emit IntPtr. The unsafe modifier drops automatically once the field is IntPtr.

---

Scoped to compatible mode only. Latest/preview (function-pointer) codegen for remapped fnptr typedefs is intentionally untouched -- there's no named delegate there, so the correct rendering is a separate, ill-defined design question.

Added FunctionPointerRemapTest covering compatible Windows + Unix. Full generator suite passes (Failed: 0, Passed: 3726), zero golden churn.

Fixes #462