Enforce resource, concurrency, and provider budgets

## Parent and milestone

- Parent issue: #43
- Milestone: not scheduled; implementation remains gated by #15.

## Problem

Provider quotas, token observations, wall time, compute, and concurrency differ
across runtimes. Without explicit budgets, failed loops can consume excessive
resources or switch providers in ways that violate operator expectations.

## Outcome

Define enforceable task, project, runtime, plugin, and node budgets with
measured-versus-estimated provenance and conservative stop behavior.

## Scope

- Define wall-time, attempt, concurrency, CPU, memory, disk, network, token,
request, and monetary budget dimensions.
- Distinguish provider-measured, runtime-reported, host-measured, and estimated
usage.
- Define warning, suspend, stop, and explicit fallback behavior.
- Require privacy, capability, and authorization compatibility before runtime
fallback.
- Expose budgets and consumption through audit and client view models.

## Non-goals

- Pretending provider subscription limits or token counters are equivalent.
- Automatically routing around provider restrictions or terms.
- Guaranteeing monetary cost when the provider exposes no reliable metric.

## Acceptance criteria

- [ ] Every enforced value records source, unit, confidence, and scope.
- [ ] Hard limits stop or suspend owner-scoped work conservatively.
- [ ] Fallback cannot weaken capability, privacy, or approval requirements.
- [ ] Restart and multi-host behavior preserve budget accounting idempotently.
- [ ] Limit races, missing metrics, retries, and stale reports are tested.

## Validation

- Deterministic fixtures for exact, delayed, estimated, missing, duplicated,
and conflicting usage observations.

## Dependencies

- #26 usage contract, #31 routing, #41 control API, and #44 resource isolation.

No implementation files or entry points are named. Start by reading dependencies #26, #31, #41, and #44, then use the acceptance criteria and validation requirements to define the budget model, fallback rules, accounting behavior, and deterministic fixtures that would constitute completion.

Written by the indexing model from the issue text.