Add bounded MediaDevices enumeration and constraints surface #773

Parent: #770
Native transport/provider: SceneTech/AppScene#289, #290, #292
Capability ledger: #263

Focused scope

Add the first browser-facing microphone slice for unchanged Code OSS:

• install branded MediaDevices and MediaDeviceInfo objects on secure top-level realms;
• implement bounded promise-based enumerateDevices() through the existing typed native host request/completion queue;
• carry only the document security origin in the request and accept a strict versioned JSON media-device payload with fixed byte/device/string limits;
• expose getSupportedConstraints() for the exact audio constraints used by Code OSS;
• expose getUserMedia() as an explicit NotSupportedError until the capture/track child lands, preventing synthetic-success claims;
• preserve request cancellation across navigation/realm teardown and reject denied/malformed/oversized host responses with browser-shaped errors;
• add WPT-derived/browser-oracle and native host ABI regression contracts without patching VS Code OSS.

Gates

Cover branding, descriptors, same-object navigator.mediaDevices, ordering, permission-redacted labels supplied by the host, bounds, malformed payloads, denial, queue pressure, stale completion and navigation teardown. Execution is deferred under the current throughput directive.