refactor(frontends): worker endpoint, metrics, and profiling ports on the protocol

[ishandhanani]

Summary

Stacked on #487. Four consumers decided which rank serves what by comparing the frontend name, each with slightly different rules: the tachometer scrape targets in core/telemetry.py (9 sites), the benchmark's logical worker endpoints, the AIPerf server metrics URLs, and the profiling control endpoints in benchmark_stage.py, plus the sequential-start readiness port in worker_stage.py. Those questions move onto the frontend protocol as metrics_path, worker_metrics_port, worker_endpoint_port, profiling_control_port, profiling_control_is_leader_only, direct_endpoint_nodes, and worker_ready_port, implemented per class from the rules each site encoded. No frontend-name comparison remains in telemetry.py or the benchmark stage. BenchmarkStageMixin.frontend is None for a services-only job.

Behavior changes

All four are corrections of dead ports the old per-site rules produced; none affects a shipped example (dry-run parity below).

• The trtllm_serve aggregate worker's logical endpoint is the public port it binds, not its unbound http_port.
• The direct sglang nsys control endpoint is the leader's public port, not http_port.
• Built-in AIPerf metrics for sglang-router target the worker leaders' HTTP ports; before, they targeted Dynamo system ports that no sglang worker serves.
• Direct frontends no longer return early from the metrics env, so KVBM and CPU power exporter URLs are appended for them like every other frontend.

Validation

• Full suite: 2972 passed on Python 3.13 and 3.10, plus 8 new contract tests in tests/test_frontends.py (a four-process topology per frontend covering every new member, and the Dynamo sidecar case).
• srtctl dry-run for all 26 example recipes is byte-identical to main after normalizing generated timestamps.
• One test fake gained the frontend_port the direct vLLM worker actually binds.

Review map

• frontends/base.py: the seven members and agg_leader_nodes.
• frontends/*.py: per-class rules. vllm_router.py overrides the two per-pool ports.
• core/telemetry.py: the per-process loop and the frontend-node selection read the frontend.
• cli/mixins/benchmark_stage.py: frontend property, _public_api_node, _logical_worker_endpoints, _profiling_worker_endpoints, _get_aiperf_server_metrics_env.
• cli/mixins/worker_stage.py: _wait_for_worker_ready.
• CLAUDE.md: Frontends section lists the members.

Still keyed on the frontend name after this PR, for the next one: _get_health_expectations and the special cases in wait_for_model, the dry-run panels in submit.py, the sglang-router metrics listener in telemetry_stage.py, and the Dynamo request-tracing checks.