Add the generated Bazel build proxy adapter

[karim-alweheshy]

Summary

• Generate the Bazel build adapter and invocation-receipt contract for schema-v2 manifests.
• Preserve native rc ordering, configuration, target IDs, output groups, working directory, and environment-key privacy.
• Complete scoped build, clean, index, and preview routing without changing non-proxy projects.

Stack

• Part of Feature Request: Have a build mode that leverages an XCBBuildService proxy for a more integrated experience #1953.
• 2 of 5 in the rules_xcodeproj build-proxy stack.
• Parent: the versioned manifest PR.
• Next: the opt-in launcher layer.

Validation

• Repository CI is green at this exact head.
• Focused manifest, adapter, receipt, and generation tests pass in the combined validation record.

Testing

Generate an opt-in project, compare the generator and proxy receipts, and verify build, scoped clean, index/preview output groups, and unchanged generation without proxy attributes.