Add an overall deadline for the complete compliance audit sequence

[viraatc]

Context

When audit: is enabled, the CLI runs the normal benchmark first (including finalize_benchmark() and accuracy scoring), then calls run_audit(). audit.only: true skips the preceding main benchmark.

PR #409 gives each AuditRunSpec subrun its own setup-inclusive run_timeout_s deadline so audit subruns match normal benchmark-run semantics. That is not an overall audit budget: a reference phase and fixed-sample phase can each consume the full value, and AuditTest.verify() runs afterward outside those per-subrun deadlines.

origin/main has no active run watchdog for audits; it only applies the existing response/metrics drain budgets inside each subrun.

Scope

Add a first-class overall audit deadline in a follow-up MR rather than overloading the per-benchmark run_timeout_s contract in PR #409. Define its ownership alongside phase configuration work in #449 and scoring/finalization cancellation work in #464.

Acceptance criteria

• One deadline begins before the first audit subrun setup and is shared across every planned audit subrun.
• Remaining budget, rather than a fresh full budget, is passed to each subrun.
• The contract explicitly covers or boundary-checks AuditTest.verify() and result writing.
• audit.only: true uses the same audit deadline semantics.
• Expiry produces a non-zero exit, honest incomplete/interrupted artifacts, and no leaked children.
• Existing per-subrun response drain, metrics drain, and interrupted teardown-grace semantics remain independently configurable.
• Integration coverage blocks deterministically during multiple audit lifecycle stages and proves the overall budget is not reset.