#4357 Two forEach steps in one job run one after the other, not in parallel

swamp 20261022.021804.0-sha.983e2929, Windows 11.

The docs (workflow/references/execution-semantics.md, "Concurrency Limits") say step-level concurrency caps that step's forEach iterations, and job-level concurrency caps parallel steps within the job. Two independent forEach steps (dependsOn: [], each on its own model, each with concurrency: 1) in one job should then overlap. They don't: every iteration of the first step runs before the second step starts. The same steps in two jobs do overlap.

Repro (a fresh repo, two command/shell models sleeper-a and sleeper-b):

jobs:
  - name: specs
    steps:
      - name: a-${{ self.i }}
        forEach: { item: i, in: "${{ [1, 2, 3] }}" }
        concurrency: 1
        task: { type: model_method, modelIdOrName: sleeper-a, methodName: execute, inputs: { run: "sleep 3" } }
        dependsOn: []
      - name: b-${{ self.i }}
        forEach: { item: i, in: "${{ [1, 2, 3] }}" }
        concurrency: 1
        task: { type: model_method, modelIdOrName: sleeper-b, methodName: execute, inputs: { run: "sleep 3" } }
        dependsOn: []

• One job: a-1, a-2, a-3 start at 18:10:46, :50, :54; b-1 only at 18:10:59. 24.7 s in all.
• The same steps as two jobs: a-1 and b-1 both start at 18:11:13. 12.3 s in all.

Expected: both forms take about 12 s. Either the step-level concurrency: 1 is applied to the whole job, or a job runs its forEach steps in sequence; if the second is intended, the docs should say so.