Proto package lacks a version suffix; no buf breaking check in CI

Severity: medium.

### Summary

The proto package is `miden_note_transport` with no `.v1` suffix (`proto/proto/miden_note_transport.proto:3`) and there is no `buf breaking` check in CI. Reflection (v1) is registered.

### Impact

For a production internet-facing API there is no path to introduce v2 side-by-side, and nothing guards against accidental wire-breaking edits (field renumbering, etc.).

### Recommendation

Rename to `miden.note_transport.v1` before GA (breaking now is cheap, later impossible) and add `buf breaking` to CI.

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Part of #114.