Add the verifier HTTP server

[AbdelStark]

What changed

This adds the verifier HTTP server described in the architecture docs.

The PR includes:

• an Axum/Tokio server with GET /health, GET /stats, POST /verify, and POST /verify/batch
• a typed request/response contract for verifier calls
• a runtime router over the existing Rust verifiers
• a small Python client for the server API
• VerifierClient(rust_server_url=...) support in the Python training harness, with local fallback when the server is unavailable or a backend is not supported by the Rust route yet
• server contract tests for health, single verification, and ordered batch verification
• a README quick start for running the server and making a verification request

Why

The repository already describes the Rust verifier server as the boundary between the Python GRPO loop and deterministic reward functions, and the Python harness already accepted a rust_server_url. The executable crate did not expose that service yet; it only printed a test hint.

This makes that boundary real. A training loop can now call a long-lived verifier process over HTTP, batch verification requests, and keep Python as the orchestration layer instead of the only verification path.

Behavior notes

• domain stays as the caller-facing domain name in the response.
• verifier is optional. When it is present, the router uses it to select the backend; this lets callers keep domain names like gsm8k while routing through math_numerical.
• Unsupported verifier names return a normal score payload with score: 0.0 and a reason, which keeps batch responses ordered and easy to consume.
• The Python harness only sends backends with implemented Rust routes to the server. Other backends keep using the existing local fallback path.

Validation

• cargo test
• cargo clippy --all-targets -- -D warnings
• python3 -m compileall -q python/rlvr
• live server smoke test with cargo run -- --addr 127.0.0.1:18080
• curl http://127.0.0.1:18080/health returned {"status":"ok","verifiers":16}
• Python client smoke test covered both single verification and ordered batch verification