perf: defer regional cache writes off the response path

[km-tr]

Fixes #1343

What

RegionalCache.set now hands both writes to ctx.waitUntil instead of awaiting them.

Why

Next.js awaits incrementalCache.set while producing the response:

Next.js  response-cache/index.js   await incrementalCache.set(...)
  └─ OpenNext adapters/cache.ts     await globalThis.incrementalCache.set(...)
       └─ RegionalCache.set          await this.store.set(...) → r2.put()

So the store round-trip lands in the TTFB. response-cache/index.js does not use the
returned value — it returns the same entry it passed in — so the writes can be deferred
without changing what is served.

The read path already defers its cache updates the same way:

// get(): refresh the regional cache in the background
getCloudflareContext().ctx.waitUntil(this.store.get(key, cacheType).then(...))
// get(): populate the regional cache after a store hit
getCloudflareContext().ctx.waitUntil(this.putToCache({ key, cacheType, entry }))

This makes set consistent with it — putToCache was previously deferred when reached
from get and awaited when reached from set.

Measured impact

Production app on Workers with the R2 incremental cache, using Workers automatic tracing.

• r2_put spans: median 976 ms, up to 2,048 ms (14 of 20 sampled traces)
• TTFB on ISR misses before the change: 2.4–4.6 s

We ran this as a local override on a staging deployment, four requests each on ISR misses:

	TTFB (median)
deferred	1.97 s
awaited	2.45 s

Repeated requests to the same page still returned x-opennext-cache: HIT at 0.32–0.69 s,
so cache behaviour is unchanged.

Trade-off

Deferring the write widens the window in which a concurrent request for the same page
misses the cache and re-renders. That window already exists for the duration of the write;
deferring shifts its start rather than creating it.

Notes

• Errors are still caught and logged inside the deferred task, so a failing write does not
  surface as an unhandled rejection.
• Added regional-cache.spec.ts covering the deferred path: set resolves while the store
  write is still pending, both writes run in the background, and a rejected store write does
  not reject the promise handed to waitUntil. The tests fail against the awaiting version.

---

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Latest commit: 56a49dc

The changes in this PR will be included in the next version bump.

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Name	Type
@opennextjs/cloudflare	Patch

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