tests: run-engine and session tests read the box's real load and time out on a busy machine

[AbirAbbas]

What happened

Several run-engine and session tests start real run seats, which pass through the machine gate (task.max_load, default 1.5 per core). On a busy box they wait machine busy · load per core at or above task.max_load 1.5, then time out. They are green in CI and alone, and red when another suite is running beside them. CLAUDE.md treats that as a bug, not a known flake.

Seen on 2026-09-25 on an 8-core box at load 11–24:

• internal/session TestUnderOneModelEveryRunSeatRidesTheConversationsModel/the_launch_model: timed out waiting for the run to end on dev@691ade547 at load 12, and 11 s alone. Added in task room, landing and --one-model: real file names, one note sentence, every seat on one model, no build caches #1517.
• internal/run TestSupervisorLaunchesEveryReadyLeafAtOnceWithNoSlotBound and TestAnUnboundedRunEndedEarlyStillDrainsEveryWorker: fail even alone at load about 11.
• Also timing out only under load: TestPlandbCliWorkerOwnDone, TestARunCorrectedPastItsRoundLimitStopsWithoutMergingOrSayingDone, TestBeltRunCarriesMachineGateAndShowsItsHold, TestPlandbCliLoopThroughBash, TestACarriedOverDesignRaisesItsCardAndSavesOnYes, TestTheDelegationDoorsCrossFromTheEnginesProfile (session); TestHostedTasksToolStopEndsTheRunAndReportsWhatHappened (remote).

Replication

Deterministic:

stress-ng --cpu $(( $(nproc) * 2 )) --timeout 600s &   # or any load above 1.5 per core
go test -count=1 -run 'TestUnderOneModelEveryRunSeatRidesTheConversationsModel' ./internal/session
go test -count=1 -run 'TestSupervisorLaunchesEveryReadyLeafAtOnceWithNoSlotBound' ./internal/run

Both fail today. Without the load, both pass.

The fix

A test that is not about the machine gate runs with the gate out of its way, through its fixture profile or an injected load reader, never the box's real /proc/loadavg. A test that IS about the gate injects the load it needs.

Acceptance

• Unit: each named test passes with stress-ng holding the box above 1.5 per core.
• A law test, or a single fixture helper, so that a new run-engine test cannot read the real load by accident.