Proto emitter: @field numbers collide when using model inheritance

When using the Protobuf emitter, @field(n) is mandatory.

However, when a model extends another model, field numbers are not
merged or offset. This leads to duplicated field numbers in the
generated message, which is invalid in Protobuf.

Example:

model Base {
@field(1)
id: string;
}

model Child extends Base {
@field(1)
name: string;
}

Generated proto:

message Child {
string id = 1;
string name = 1; // invalid: duplicate field number
}

This makes model inheritance unsafe with Protobuf.

Current issues:
- @field numbers are scoped per model, not per message
- No validation or error is raised
- No automatic offset or merging strategy exists

This breaks:
- Protobuf constraints
- Backward compatibility guarantees
- Design-first modeling with inheritance

Suggested directions:
- Automatically shift child field numbers after inherited fields
- Or support an implicit field numbering strategy
- Or disallow inheritance for proto models with a clear error

At the moment, using inheritance with Protobuf is dangerous and
non-deterministic.