refactor(stella-core): take the two shared sleeper doubles from stella-time

[macanderson]

What & why

The test-double half of #6484, on top of the merged stella-time PR (#6488). Re-opened: the first copy (#6491) was closed by GitHub when #6488's branch was deleted on merge. About thirty test files across stella-core, stella-engine and their tests/ directories each wrote the same Sleeper double — a no-op one under three names, and one on tokio's paused clock — and each copy grew a now() body in #6486. Every one of them is deleted here.

• Integration tests and stella-engine take PausedSleeper and NoopSleeper from stella_time::test_util through a dev-dependency on stella-time with test-util on. For stella-core that is a dependency cycle, which cargo allows; it is the tokio / tokio-test shape.
• stella-core's own unit tests keep one copy, src/tests.rs, because a lib's unit tests are a second build of the lib and a dev-dependency that links the lib implements the trait for the first. The compiler forces that copy (the first attempt at taking the crate's doubles from stella-time failed on every unit test with "multiple different versions of crate stella_core"). It is named tests.rs so make core-no-io reads it as test code, which is how that guard tells test code from shipping code.
• The one_home witness in stella-time now scans test files too. Its kept list shrinks to the four doubles with a shape a shared one cannot have — the retry tests' recording sleeper, the monitor's advancing sleeper, the hanging sleeper, and step/tests.rs's unpaused-time sleeper — plus the forced copy.

Closes #6484

The witness

• This PR includes a witness test (fails on main, passes here)

crates/stella-time/tests/one_home.rs reads every .rs under crates/*/src and crates/*/tests. On the base branch it fails on twenty-three files that each define a sleeper double; here it passes, and a second test fails if a kept one disappears. Locally: stella-core's 893 lib tests and every integration test, stella-engine's tests and the time crate's 6 + 3 tests pass.

The gate

• cargo fmt --check
• clippy over the workspace at -D warnings — CI; ran locally on stella-core, stella-engine, stella-time
• the workspace test suite — CI
• Docs updated where behavior/flags changed (README, --help, doc comments)
• CLA signed
• Closes #N appears both above and as a commit trailer

Fix over file

• Extra fixes in this PR: none beyond the sweep
• Nothing was deferred

Ground-rule check

• No I/O added to stella-core; src/tests.rs is #[cfg(test)] and make core-no-io is green
• No new outbound network calls
• No new cross-boundary serde types

Anything reviewers should know?

• The rename TokioSleeper → PausedSleeper in the test tree is deliberate: the paused-clock double must not share a name with the real stella_time::TokioSleeper.
• Once fix(scripts): read a #[cfg(test)] mod line as test code, and drop sharp from the docs site #6498 lands (the no-io guard reads #[cfg(test)] mod lines), src/tests.rs could take a more descriptive name; it is named for the guard's current file-name rule.

Summary by Sourcery

Consolidate test sleeper doubles in stella-time and enforce a single ownership location across the workspace.

Enhancements:

• Centralize the shared no-op and paused-clock sleeper doubles in stella-time and update stella-core and stella-engine tests to use them.
• Retain only the specialized sleeper doubles that require unique behavior, including the compiler-required stella-core unit-test copy.
• Extend the sleeper ownership witness to scan test sources and document the shared test utility arrangement.

Build:

• Add stella-time as a test-only dependency with the test-util feature where shared sleeper doubles are used.

Documentation:

• Update architecture and crate documentation to describe the shared sleeper test utilities and the stella-core unit-test exception.

Tests:

• Update the affected unit and integration tests to use the centralized sleeper doubles.

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Reviewer's Guide

This refactor removes duplicated sleeper test doubles from stella-core, stella-engine, and their test suites, uses the shared stella-time::test_util implementations wherever Cargo permits, keeps one stella-core unit-test copy required by Rust’s separate library test build, and adds a source-scanning witness to prevent future duplication.

Flow diagram for the sleeper duplication witness

flowchart TD
    Scan["one_home.rs scans crate src and tests .rs files"] --> Detect["Find sleeper double definitions"]
    Detect --> Allowed["Compare against kept-list"]
    Allowed --> Pass["Pass when shared doubles are used and required exceptions remain"]
    Allowed --> Fail["Fail on duplicated or missing sleeper doubles"]

Loading

File-Level Changes

Change	Details	Files
Centralize the standard no-op and paused-clock sleeper doubles in stella-time and migrate tests to use them.	Enable the test-util feature through dev-dependencies in stella-core and stella-engine. Replace locally defined sleeper implementations across unit and integration tests with stella_time::test_util::{NoopSleeper, PausedSleeper}. Rename paused-clock test usage from TokioSleeper to PausedSleeper to distinguish it from the production sleeper.	crates/stella-core/Cargo.toml crates/stella-core/tests/engine_emits_no_stage.rs crates/stella-core/tests/hard_drop_write_back.rs crates/stella-core/tests/parallel_dispatch.rs crates/stella-core/tests/spend_gate.rs crates/stella-core/tests/tool_wall_clock.rs crates/stella-engine/Cargo.toml crates/stella-engine/src/tests.rs crates/stella-engine/tests/embedding.rs crates/stella-core/src/accounted_call.rs crates/stella-core/src/driver/ crates/stella-core/src/goal.rs crates/stella-core/src/subagent/
Retain the compiler-required sleeper implementations for stella-core library unit tests in a dedicated test-only module.	Add src/tests.rs with PausedSleeper and NoopSleeper implementations matching the shared test utilities. Expose the module only under #[cfg(test)] and update unit-test imports and call sites. Document why the unit-test build cannot use the dev-dependency's implementations.	crates/stella-core/src/lib.rs crates/stella-core/src/tests.rs crates/stella-core/src/subagent/tests.rs crates/stella-core/src/subagent/tests/*.rs
Expand the sleeper ownership witness to cover test sources and enforce the remaining intentional exceptions.	Scan both src and tests Rust files across workspace crates. Remove retired sleeper copies from the allowlist and retain only the forced unit-test copy and doubles with specialized behavior. Update witness documentation and the expected kept paths.	crates/stella-time/tests/one_home.rs
Update architecture and crate documentation to describe shared test utilities and the allowed dependency-cycle arrangement.	Document stella-core and stella-engine test usage of stella-time::test_util. Clarify the unit-test exception and dev-dependency cycle in the crate README, ADR, and repository guidance. Refresh test comments and nearby prose without changing production behavior.	AGENTS.md crates/stella-core/README.md crates/stella-time/README.md docs/adr/0042-the-engine-reads-time-through-the-sleeper-port.md crates/stella-core/src/accounted_call.rs

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#6484	Provide a single shared home for the sleeper test doubles, with PausedSleeper and NoopSleeper behind stella-time's test-util feature, and remove the duplicated doubles from stella-core, stella-engine, and their integration tests except for necessary specialized or compiler-forced cases.	✅
#6484	Add an automated witness that scans the workspace for duplicate sleeper implementations and documents the specific exceptions that are intentionally retained.	✅
#6484	Complete the time-source consolidation by using stella-time as the shared home for production clock and sleeper implementations, while preserving the no-I/O constraint of stella-core and documenting the design.	✅

Possibly linked issues

• The engine reads the clock itself in nineteen places, and three crates keep their own copy of the wall clock #6484: PR removes duplicated sleeper implementations, adopts stella-time test utilities, and adds witness coverage required by the issue.

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