Start HEAD freshness after revalidation

[Jason-Doyle]

Summary

• start mutable HEAD freshness when a successful 304 Not Modified response completes
• cover slow revalidation for both Trie and Snapshot browser reads
• document the response-completion TTL semantics and add the fix to the changelog

The adaptive backoff experiment remains isolated and is not included in this change.

Validation

• npm test
• npm run check
• npm run build
• npm run verify:package
• npm run verify:docs
• npm run test:e2e

Regional browser evidence supporting the timing correction remains on experiment/adaptive-head-revalidation.