[crowlbot]

This implements the User-Agent Client Hints API — navigator.userAgentData, returning a NavigatorUAData object — as requested in #16830.

navigator.userAgentData is exposed in both the window and worker global scopes, alongside the NavigatorUAData global interface.

Supported surface

• Low-entropy properties: brands, mobile, platform
• getHighEntropyValues(hints) — resolves with the requested hints plus the always-present low-entropy values. Supported hints: architecture, bitness, brands, formFactors, fullVersionList, mobile, model, platform, platformVersion, uaFullVersion, wow64. Unknown hints are ignored, per spec.
• toJSON() — returns the low-entropy values.

Behavior notes

• brands reports Deno with the major version; fullVersionList/uaFullVersion report the full x.y.z version, all derived from the runtime's user-agent string.
• platform maps the build OS to the browser-style name ("Linux", "macOS", "Windows", …); architecture maps the build arch (x86_64 → "x86", aarch64 → "arm").
• mobile is always false and bitness is "64", matching how Deno targets are built.
• The constructor is illegal (cannot be invoked directly), matching browsers.

Example

navigator.userAgentData.brands;
// [ { brand: "Deno", version: "2" } ]

navigator.userAgentData.platform;
// "Linux"

await navigator.userAgentData.getHighEntropyValues(["architecture", "uaFullVersion"]);
// { brands: [...], mobile: false, platform: "Linux", architecture: "x86", uaFullVersion: "2.8.1" }

Tests are added in tests/unit/navigator_test.ts covering the low-entropy values, getHighEntropyValues (including unknown/empty hints and the missing-argument rejection), toJSON, the illegal constructor, and custom inspect output.

Closes #16830