Preserve stable Protobuf field tags and support field lifecycle management

[stelund]

Preserve stable Protobuf field tags and support field lifecycle management

Summary

This PR makes IFEX-to-Protobuf generation safe for long-lived, wire-compatible
APIs.

It preserves field numbers when parsing and rendering .proto files, requires
explicit stable tags for IFEX-generated fields, and adds support for deprecated
and removed struct members.

Changes

• Preserve regular and map-field numbers in the Protobuf AST and round-trip output.
• Render actual Protobuf field numbers instead of the previous hard-coded = 0.
• Add protobuf_tag to IFEX struct members and method input/output/return arguments.
• Validate generated messages before rendering:
  • tags are required;
  • tags are unique per message;
  • tags must be within 1..536870911;
  • tags 19000..19999 are rejected. (Reserved by protobuf)
• Prevent declaration order from assigning wire identifiers: inserting or reordering fields without explicit tags now fails generation.
• Add protobuf_deprecated: true, emitted as [deprecated = true] while retaining the existing tag.
• Add struct-level protobuf_reserved_tags and protobuf_reserved_names, emitted as valid Protobuf reserved statements.
• Reject active struct members that conflict with a reserved tag or name.
• Document the stable-tag, deprecation, and removal policy in the Protobuf mapping guide.

Example

structs:
  - name: VehicleStatus
    protobuf_reserved_tags: [2]
    protobuf_reserved_names: [legacy_state]
    members:
      - name: speed
        datatype: uint32
        protobuf_tag: 1
      - name: old_state
        datatype: string
        protobuf_tag: 3
        protobuf_deprecated: true

Generates:

message VehicleStatus {
  uint32 speed = 1;
  string old_state = 3 [deprecated = true];
  reserved 2;
  reserved "legacy_state";
}

Compatibility / migration impact

This intentionally makes missing protobuf_tag a generation error. Existing
IFEX definitions that generate Protobuf must add explicit tags before
generation. Tags must never be renumbered or reused after publication.

Validation

Tested with repository-local .venv on Python 3.12:

8 passed

Commits included:

1. Preserve protobuf field numbers
2. Require stable protobuf field tags
3. Support deprecated and reserved protobuf fields

[stelund]

If this is the path for defining protobuf tags, I have some more improvements for better protobuf generations.

If someone has another idea how to describe it, I would be all ears.