feat: add W&B worker GPU system metrics

[EazyReal]

Summary

In single-controller training the process that calls wandb.init() runs on a
CPU/controller host while actor and rollout workers run on GPU nodes. W&B
collects system metrics from the initialized W&B client, so wandb.mode=online
on only the controller records controller-process memory but not worker GPU
utilisation.

This PR opts workers in to W&B system metrics via shared-mode:

• Controller initialises the primary run with x_primary=True.
• Selected worker processes attach a non-primary client with a role/rank
  x_label, x_primary=False, and x_update_finish_state=False, so they
  contribute system metrics only and never finalise the primary run.
• Worker init is locked, exception-tolerant, and a no-op when disabled.

Supersedes #1335 (the previous attempt was closed before the head branch could
be reopened).

Usage

stats_logger:
  wandb:
    mode: shared
    system_metrics:
      enabled: true
      roles: [actor, rollout]   # null = every configured role
      gpu_device_ids: null      # let W&B use the worker's visible GPUs

Hardening since #1335

• WandBSystemMetricsConfig.roles is now list[str] | None, normalized from
  tuples in __post_init__. Empty roles and negative GPU ids are rejected.
• init_worker_wandb_system_metrics is protected by a threading.Lock and
  catches wandb.init failures so worker telemetry can never crash training.
• finish_worker_wandb_system_metrics is exception-tolerant and idempotent.
• New unit tests cover the validators, tuple to list normalization, and the
  init-failure path. Logger color registered for WandBSystemMetrics.

Validation

• uv run pre-commit run --from-ref main --to-ref HEAD — all hooks pass
  (mdformat, ruff lint+format, conventional-pre-commit, CLI doc regen, SPDX).
• uv run python -m pytest tests/test_wandb_system_metrics.py tests/test_trackio_backend.py
  — 20 passed.

Test plan

• safe-to-test CI on a 2-GPU runner.
• Optional: run an end-to-end shared-mode job and confirm worker GPU
  utilisation appears alongside the controller run.