fix(lru): actually evict stale entries from TtlLruCache on lookup

[Tatlatat]

What

TtlLruCache is documented as "a stale hit is treated as a miss and
evicted", but get() didn't evict:

get(key: K): V | undefined {
  const e = this.inner.get(key);            // already promotes the entry to MRU
  if (!e) return undefined;
  if (e.expiresAt <= Date.now()) return undefined;  // miss — but entry stays
  return e.v;
}

Two problems for an expired key:

1. It is never removed — it lingers in the underlying map.
2. The preceding this.inner.get(key) has already re-inserted it as
   most-recently-used, so the stale entry outlives fresher ones during
   eviction.

TtlLruCache backs the gitignore cache and the directory-listing cache (both
5s TTL), which are hit on hot file-operation paths — so stale entries
accumulate and skew eviction under load.

Fix

• Add a public delete(key) to LruCache.
• In TtlLruCache.get, call this.inner.delete(key) when the entry is expired
  before returning undefined, so it is evicted immediately as documented.

Test

Appended a TtlLruCache case (fake timers): after the TTL elapses, a lookup
returns undefined, the key is gone from the underlying store, and it no longer
survives subsequent evictions. Verified it fails on the old code and passes on
the fix.

Verification

npx vitest run tests/lru.test.ts (11 passed); biome check clean on the
changed files.