Ambient host conditions

std/host_conditions exposes a read-only snapshot of ambient contention:

import { host_conditions_sample } from "std/host_conditions"

const snapshot = host_conditions_sample()
for answer in snapshot.questions {
  log(answer.question, answer.status, answer.contention)
}

The contract names questions rather than platform signals:

Question	Meaning
`promised_cpu`	Is this workload receiving the CPU capacity it was promised?
`nominal_speed`	Is execution running below nominal speed?
`accelerator_shared`	Is the accelerator allocation shared?
`memory_or_io_contended`	Is memory or storage pressure delaying work?

Each answer has exactly one observability state:

observed includes contention from 0.0 (a real quiet reading) through 1.0 (maximum normalized contention).
unavailable means the environment should expose the answer but this read failed. Retry or alert on repeated failures.
not_observable means the environment cannot expose the answer. Do not retry forever or treat it as quiet. For example, a guest cannot observe host thermal throttling, so nominal_speed is not_observable, never an observed zero.

Callers threshold the question and do not branch on Linux, VM, container, or signal names. The local backend uses load relative to available CPUs on ordinary hosts, CPU steal in virtual guests, cgroup quota/throttling in containers, CPU frequency where visible, and Linux memory/IO pressure counters. Missing accelerator allocation metadata is reported honestly rather than inferred.

Managed embedders can install HostConditionsCapability::with_source(...) with a HostConditionsSource. The injected source returns the same HostConditionsSnapshot type as local probing, so Harn callers keep the same host_conditions_sample() call while a control plane supplies richer facts. The capability validates the version, the complete four-question set, and state/value invariants at that boundary.

Sampling reads a small, bounded set of in-memory kernel files and starts no processes or network requests. sample_cost_us records the actual end-to-end cost of every sample; the intended cost is microseconds to low milliseconds, small enough to sample for every unit of measurement.

Correctness comes from assignment

Randomized assignment plus blocking on host is the correctness guarantee for comparisons. It works even when the environment exposes no contention facts. Ambient conditions are only an optimization and attribution aid: they can avoid wasting trials on a loud host and explain suspicious measurements. They never substitute for randomized assignment. Without randomized assignment, telemetry can make a biased comparison look controlled; without telemetry, randomized assignment remains correct but may require more trials.