Promote warmup to a first-class phase type with per-phase config

Follow-up from PR #409 review (suggested by @arekay-nv in https://github.com/mlcommons/endpoints/pull/409#discussion_r3725383602).

Today warmup is a special-cased pre-phase: its knobs are scattered (settings.warmup block, settings.timeouts.warmup_drain_timeout_s) and phases are implicit. Proposal:

• Explicit phase types with declared ordering/dependencies (warmup before performance, reporting after accuracy, ...).
• Per-phase config blocks owning that phase's knobs (sample count, drain timeout, salt/seeds), replacing the flat per-phase drain fields in settings.timeouts.
• The whole-run run_timeout_s watchdog (landed in #409) stays the only global deadline.

The Timeouts per-phase drain fields map 1:1 onto per-phase blocks when this lands.

Start by reviewing PR #409 and tracing the current settings.warmup block and settings.timeouts per-phase drain fields. Define explicit phase types, ordering and dependencies, then assign each phase its configuration while keeping run_timeout_s as the global watchdog; done means the warmup and existing phases use the new structure without the scattered fields.