Link cache-hits to the trace that filled the cache entry

*Example:* a product page uses

```
async function getProducts() {
'use cache';
cacheLife('hours');
return db.query('SELECT * FROM products');
}
```

Visitor A at 09:00 misses. The DB query runs, 800 ms, trace A. Visitor B at 14:00 gets a 2 ms hit and complains about yesterday's price. B's trace shows a fast green `cache.get` and nothing else. Trace A, where the content came from, cannot be found from B.

*Solves:* you can navigate from any cache hit to the render that produced the content. One bad render can serve thousands of visitors, and today the render is the one trace you cannot reach.

*Fix idea:* at handler `set`, store the active trace context keyed by cache-key digest (mechanism per the transport decision, 3.2). At a `get` hit, always attach a real span link: `span.addLink()` with `sentry.link.type: 'cache_origin'` pointing at the fill span. The link is unconditional and the durable data model; it renders in the span drawer today and needs no SDK change once Sentry stores links first-class. Separately, duplicate it into the flat bridge attribute, but only if the backend meeting lands on "EAP links take too long". The prototype verified the link round-trips. Acceptance: an E2E miss/hit pair shows the span link on the hit transaction, independent of the bridge attribute.

Start by locating the cache handler's set and get paths and reviewing the transport decision in section 3.2, then inspect the existing prototype's round-trip behavior. Add an end-to-end miss/hit case and verify that the hit transaction shows a cache_origin span link independently of the flat bridge attribute.

Written by the indexing model from the issue text.