Add a versioned build proxy manifest

[karim-alweheshy]

Summary

• Add a deterministic schema-v2 build-proxy manifest to generated Xcode projects.
• Bind project identity, target GUIDs, Bazel labels, target IDs, variants, operation capabilities, and output groups.
• Keep projects without proxy configuration unchanged.

Stack

• Part of Feature Request: Have a build mode that leverages an XCBBuildService proxy for a more integrated experience #1953.
• 1 of 5 in the rules_xcodeproj build-proxy stack.
• Parent: main.
• Next: the generated Bazel adapter layer.

Validation

• git diff --check passes at this exact endpoint.
• The combined stack passes the repository test matrix; exact-layer CI is expected on this draft.

Testing

Run the manifest analysis and generation tests, regenerate the Sanitizers fixture twice, and verify byte-identical schema-v2 output plus unchanged generation when proxy attributes are absent.