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.

[karim-alweheshy]

Rebased and normalized the complete build-proxy review stack onto current main.

For this root slice specifically, the old branch-only dependency/archive duplicates were dropped where already present on main; the three actual manifest commits were replayed patch-identically. The resulting head is 626d5f9d9dde4d9717c7145c992946bef0e0dd68 and GitHub reports it mergeable against current main.

The downstream review chain is now a single linear 23-commit DAG through #3339. All 12 intended edge relationships, changed-path sets, commit counts, dropped duplicates, and stable patch IDs were mechanically verified. Four focused build-proxy test targets pass on the final endpoint.

Because this repository uses squash-only merges, I am marking only this root slice ready. The downstream slices will remain draft and mechanically mergeable against their updated stack bases until each parent is promoted; then they can be retargeted/replayed one at a time.