Proposal: Add optional threadId to retryAttempts entries #74

Problem

Entries in retryAttempts have their own start and stop, but no threadId. The test object's threadId describes the final attempt (§9.3), so a consumer knows when an earlier attempt ran, but not on which thread or worker.

Timeline views need this. Today they have to guess, usually the final attempt's thread, and the guess is often wrong:

• Gradle test-retry runs retries in a second round, with new test JVMs.
• Playwright discards the worker process after a failure and runs the retry in a new one. It already reports workerIndex and parallelIndex on each TestResult, i.e. per attempt.

Producers can't add it themselves: §11 forbids unknown fields outside extra, and the schema sets additionalProperties: false on attempt items.

Proposal

1. Add an OPTIONAL threadId (string) to the Attempt History Entry Object (§11): "Identifies the thread or worker on which this attempt executed." Same format as the test object's threadId.
2. Reword §9.25 to "on which the final attempt executed".
3. If an attempt entry has no threadId, its thread is unknown; it is not implied to be the test object's threadId. (Inheriting would make every existing report claim that earlier attempts ran on the final attempt's thread.)
4. Within a document, a threadId value SHOULD identify one worker, so executions that share it ran one at a time. Producers that run tests in several processes or on several hosts SHOULD qualify it, for example fork-2/Test worker.

Schema, in retryAttempts.items.properties:

"threadId": {
  "description": "Thread or worker on which this attempt executed",
  "type": "string"
}

Example: a test that failed on one Gradle fork and passed on retry on another.

{
  "name": "checkout accepts a saved card",
  "status": "passed",
  "duration": 1200,
  "start": 1761200410000,
  "stop": 1761200411200,
  "threadId": "fork-2/Test worker",
  "retries": 1,
  "retryAttempts": [
    {
      "attempt": 1,
      "status": "failed",
      "duration": 1500,
      "start": 1761200400000,
      "stop": 1761200401500,
      "threadId": "fork-1/Test worker"
    }
  ]
}

All of this is additive; every valid 0.2.0 document stays valid.

Context

I ran into this while building a timeline report for github-test-reporter (ctrf-io/github-test-reporter#279). Related: #58 (retry model, settled in #62), #57 and #73 (attempt identity). Happy to open the spec and schema PR.