feat: add parallel bandwidth measurements

[andre-j3sus]

Adds configurable bandwidth origins and numeric parallelism at both the test and individual-step levels. Bandwidth requests run in bounded batches, with aggregate bytes and throughput calculated across each overlapping transfer window and reported through the existing results payload.

Sequential execution remains the default, while physical-request logging stays per request and final session metadata records the expected maximum parallelism.

new SpeedTest({
  bandwidthOrigins: [
    "https://speed-0.example.com",
    "https://speed-1.example.com",
    "https://speed-2.example.com",
    "https://speed-3.example.com"
  ],
  parallelism: 4,
  measurements: [
    { type: "download", bytes: 1e7, count: 8 },
    { type: "upload", bytes: 1e7, count: 8 },
    { type: "download", bytes: 2.5e7, count: 2, parallelism: 1 }
  ]
});

[ask-bonk]

LGTM!

github run

[andre-j3sus]

API design question: The current API uses complete endpoint URLs:

downloadApiUrl: "https://speed.cloudflare.com/__down"
uploadApiUrl: "https://speed.cloudflare.com/__up"

This proposal adds bandwidthOrigins and derives /__down and /__up automatically:

bandwidthOrigins: ["https://speed-0.example.com"]

To remain consistent with the existing full-URL approach, alternatives would be paired endpoints:

bandwidthEndpoints: [{ download: ".../__down", upload: ".../__up" }]

or separate downloadApiUrls and uploadApiUrls arrays. Reviewers, which contract do you prefer?