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

[viraatc]

Follow-up from PR #409 review (suggested by @arekay-nv in #409 (comment)).

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 feat(config): consolidate all time knobs into settings.timeouts; make --timeout a real run watchdog #409) stays the only global deadline.

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

[arekay-nv]

Can we also fix the salt implementation so that the warmup phase can check if salting is requested, and validate whether the dataset supports salting. Each dataset can then maintain its salting mechanisms since the right way to salt a request is domain dependent.