Add Burrito release packaging for Symphony

#### Context

Using Symphony currently requires installing and configuring an Elixir runtime. Downloadable executables make first use much simpler.

#### TL;DR

*Add self-contained Burrito executables for macOS and Linux.*

#### Summary

- Add Burrito targets for macOS and Linux on arm64 and x86_64.
- Route packaged releases through the existing Symphony CLI.
- Build release assets, run native smoke tests, and publish checksums from GitHub Actions.
- Document release binaries and source-run examples.

#### Alternatives

- Escript still requires Erlang/Elixir on the user's machine.
- Native macOS Burrito builds hit a Zig linker failure, so CI cross-builds on Linux.
- The workflow keeps the existing acknowledgement gate but does not pass the flag itself.

#### Test Plan

- [x] `make -C elixir all`
- [x] `mise exec -- mix test --cover`
- [x] `mise exec -- mix lint`
- [x] `mise exec -- mix dialyzer --format short`
- [x] `actionlint .github/workflows/burrito-release.yml`
- [x] Run the packaged macOS arm64 CLI on this Mac.