Docs updates #32

Problem

The log backend examples in README and DESIGN.md, which apps copy, can silently lose records:

• keepalive: true on every request. Browsers refuse a keepalive request once all keepalive requests and beacons in flight exceed 64 KiB, so a burst of log records is dropped (the fetch rejects, and the example swallows it).
• sendBeacon for every batch (DESIGN.md's batching example). sendBeacon has the same 64 KiB quota and returns false when it refuses; the example ignores the result, so the batch is lost.

Expected

• keepalive only while the page is hidden, i.e. may be closing (document.visibilityState === 'hidden').
• The batching example sends with a plain fetch while the page is visible, uses sendBeacon only in flush(), and falls back to fetch when the beacon is refused, documented as best effort on unload.
• README and DESIGN.md say why, briefly.