Parse the wall ceiling from the budget manifest map (budget: { wall: <dur> })
and arm it alongside the per-run timeout. Whichever ceiling fires first binds;
the kill is recorded as result=timeout or result=budget_exceeded accordingly.
budget.wall extends timeout: a job with only a budget.wall (no timeout) is now
hard-killed at the ceiling. budget_exceeded is a terminal, non-retryable class
by default and maps to the failed tracker status.
Adds _budget_wall_secs + _effective_kill helpers (pure, unit-tested) and live
selftest coverage; usd/tokens remain best-effort and are not enforced here.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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