perf: TTL-based eviction sweep for prefetch cache

[Divkix]

Problem

The prefetch cache in navigation.ts stores RSC prefetch responses with a 30s TTL and a hard cap of 50 entries. Expired entries are only cleaned on individual access or displaced by FIFO when at capacity. This means expired entries waste slots, causing premature eviction of still-valid entries on link-heavy pages.

Solution

Sweep all expired entries before resorting to FIFO eviction in storePrefetchResponse(). This ensures fresh entries survive longer under pressure without increasing the cache size cap.

Implementation

PR: #434