Add Beacon API via navigator.sendBeacon

Proposal

Implement navigator.sendBeacon for fire-and-forget telemetry and background-safe submissions.

Scope:

• Expose navigator.sendBeacon with URL and body inputs.
• Support string, blob, and byte-like body forms.
• Return boolean based on acceptance of enqueue/send.
  Technical Notes:
• Route through requesters/loader with non-blocking semantics.
• Inject policy for size limits and batching if needed.
  Acceptance Criteria:
• sendBeacon returns true for accepted payloads and false for rejected ones.
• Body serialization and content type handling are covered by tests.
• No unhandled exceptions on unsupported payloads.
  Out of Scope:
• Browser-specific background delivery guarantees.
• Retry persistence across process restarts.

Description

Implement navigator.sendBeacon as a lightweight, non-blocking telemetry submission API with body type support and deterministic acceptance behavior.

Background

sendBeacon is widely used for analytics and end-of-session events where fire-and-forget semantics are preferred over fetch/XHR lifecycle coupling.

Specification

• Beacon specification: https://w3c.github.io/beacon/
• API surface: navigator.sendBeacon(url, data)

Start by locating the navigator API surface and the requesters/loader path mentioned in the issue. Trace how URL and body inputs are serialized and how non-blocking requests are accepted or rejected. Done means sendBeacon supports the listed body forms, deterministic boolean results, content-type and serialization tests, and no unhandled exceptions for unsupported payloads.