126Implement an HTTP cache with ETag validation, stale-while-revalidate, and request deduplication

Implement a client-side HTTP cache that follows the server's caching headers: createHttpCache({ fetch, now = Date.now }) returns { get(url) }, which resolves with { status, body, fromCache }.

The server sends ETag and Cache-Control headers, for example Cache-Control: max-age=60, stale-while-revalidate=300.

Rules for get(url):

• No entry: fetch, and store { body, etag, storedAt, maxAge, swr } unless the response has Cache-Control: no-store. Only cache 200 responses.
• Fresh (age < max-age seconds): return the cached body without a request (fromCache: true).
• Stale but within stale-while-revalidate: return the cached body immediately and revalidate in the background.
• Older than that: revalidate and wait for the result.
• Revalidating sends If-None-Match: <etag> when there is an ETag. A 304 Not Modified keeps the cached body and resets its age; a 200 replaces the entry.
• Deduplication: concurrent gets for the same URL that need the network share one request.
• A failed background revalidation keeps the stale entry; a failed blocking request rejects.

fetch(url, { headers }) resolves with a standard Response. Body text can be read with await response.text().