feat: per-source env-server address via serve.address

[mikasenghaas]

Summary

• The per-source [serve] block is now verifiers' ServeConfig, consumed as-is — no prime-rl mirror or subclass. feat: make ServeConfig.address optional with no default verifiers#2301 (merged) makes ServeConfig.address optional with no default (str | None = None); this PR bumps deps/verifiers to that rev (29e3a0f7a, current verifiers main).
• Unset address (default) keeps today's behavior: the launcher serves the source at the derived tcp://127.0.0.1:<env_server_base_port + index> address.
• Set, the source's env server is externally managed: the launchers (local and SLURM) neither write its env-server TOML nor spawn a server for it, and the orchestrator connects to the given address — e.g. a k8s deployment running each env server in its own pod sets serve.address per source.
• Indices stay positional across all sources, so setting one source's address never shifts another's derived port.
• Standalone uv run env-server without an address keeps binding tcp://127.0.0.1:5000 (serve_env falls back to its default loopback bind on None).

Supersedes #3216: the same capability expressed on the source itself instead of a launcher-owned top-level map — no <split>/<resolved_name> string keys, and no key-typo validator needed since the field lives on the source it applies to.

Verification

Sanity-checked with the main venv against this branch and the new verifiers rev: a 3-source config (train/a, train/b with serve.address = "tcp://env-b.svc:5000", eval/c) yields env_addresses of {a: tcp://127.0.0.1:5000, b: tcp://env-b.svc:5000, c: tcp://127.0.0.1:5002}, env_servers() skips b, and the serialized orchestrator TOML round-trips the address while unset addresses don't serialize.

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