Add CI for phoenix/ (Python tests + Rust workspace check)

[AnonRish]

Summary

• Add a GitHub Actions workflow scoped to phoenix/, the one component
  documented as buildable outside X's internal infrastructure
• python-tests: uv sync --group dev + pytest over xrex
• rust-check: installs the workspace's declared MSRV, the documented
  system packages, and protoc, then cargo check --workspace --all-targets
  and cargo test --workspace

Problem

No GitHub Actions workflow exists anywhere in this repository. Every
currently open PR is relying entirely on manual review with no automated
signal.

Fix

A minimal, two-job workflow triggered on changes under phoenix/:

• python-tests runs the pure-Python surface only (no Rust toolchain
  required), keeping it fast and independent of the Rust build.
• rust-check pins rustc 1.85 explicitly (this workspace's declared
  minimum, not whatever the runner ships today) and installs the same
  system packages and protoc version documented in the README.

home-mixer and the other services are intentionally excluded: none of
them has a standalone Cargo.toml or build manifest in this repository, and
they import internal-only packages, so there's nothing yet for a public
runner to build. cargo fmt --check and clippy are also deliberately
excluded from this PR -- unclear whether existing code is clean against
either, and a first CI PR shouldn't open red against pre-existing code;
both are reasonable fast-follows once this is green.

Verification

• YAML validated with python -c "import yaml; yaml.safe_load(open(...))".
• python-tests commands run end to end: uv sync --group dev && uv run pytest passes 17/17 against the suite added in the companion tests PR.
• rust-check could not be run locally (no rustc >=1.85 reachable in this
  environment); the system-package list and protoc invocation are copied
  from what's already verified working in phoenix/README.md's own
  instructions, corrected to linux-x86_64 for this runner's architecture.

Please confirm the rust-check job goes green before merging, or flag
from CI logs if it doesn't -- happy to iterate.