Allow `C.S{}` zero-init for C-imported structs containing reference fields

[eptx]

Environment

• V version: V 0.5.1 99f141f74 (verified no relevant commits land between that and current master b6dfae560)
• OS-agnostic

Reproducer

main.v:

module main

struct C.MyStruct {
mut:
    format       &char
    name         &char
    children     &voidptr
    release      fn (&C.MyStruct)
    private_data voidptr
}

fn main() {
    s := C.MyStruct{}   // ← fails
    _ := s
}

V 0.5.1 (and master b6dfae560) errors:

error: reference field `C.MyStruct.format` must be initialized
error: reference field `C.MyStruct.name` must be initialized
error: reference field `C.MyStruct.children` must be initialized

Why this matters

C-imported structs (struct C.X { … }) frequently match real C ABI shapes where reference-typed fields are expected to start as NULL and be filled in by the caller. The Arrow C-Data interface (ArrowSchema, ArrowArray) is the canonical example: every consumer in every ecosystem allocates the struct, zeroes it, then fills the fields. The C convention assumes {0} initialisation.

V's check is correct for V-native structs (a &T field nil-by-default is unsound). For C-imported structs the check is too strict: the user is opting into C ABI semantics and should be trusted, much as unsafe blocks already trust the user for raw pointer arithmetic.

Workaround used today

CX's vcx/arrow/arrow.v:393-410:

child_struct_size := int(sizeof(C.ArrowSchema))
cs_raw := unsafe { malloc(child_struct_size) }
unsafe { vmemset(cs_raw, 0, child_struct_size) }
mut cs := unsafe { &C.ArrowSchema(cs_raw) }
cs.format = c_string(col_formats[i])
... // fill each field

The memory layout is identical to what C.ArrowSchema{} would produce; V just won't let the type-system express it.

Proposed fixes

In order of disruption:

1. Skip the check for struct C.… types. They're already in the "trust the C ABI" lane.
2. Allow unsafe { C.S{} } as a sanctioned escape hatch.
3. Allow C.S{ ...fields_set_to_unsafe_nil } — verbose but explicit.

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