Function invalidateMappingCache

invalidateMappingCache(key?: string): Promise<void>
Drops cached shard mappings so the next read resolves them from KV again.

CollegeDB caches a key's shard in memory for mappingCacheTtlMs, which makes that TTL a staleness window: a mapping changed elsewhere, by another isolate or by a KVShardMapper built directly, is not visible here until the entry expires. reassignShard already clears what it changes, so this is for changes made outside CollegeDB's own routing.

Re-running initialize is not a substitute. It reuses the existing mapper when the bindings and options are unchanged, precisely so the cache survives the per-request initialization the Workers examples use.
Parameters
- Optionalkey: string
  Logical key to drop, or omit to drop every cached mapping
Returns Promise<void>
Throws
If CollegeDB is not initialized

Since
1.4.0
Example
```
const mapper = new KVShardMapper(env.KV, { hashShardMappings: true });
await mapper.updateShardMapping('user-123', 'db-west');

invalidateMappingCache('user-123'); // this process now reads the new shard
```
- Defined in src/router.ts:2018