Kevin Guthrie

make Lru::shard_weight a lock-free read Add a per-shard atomic shadow of total weight (shard_weights: [AtomicUsize; N]) maintained alongside the existing global weight counter at every weight-mutating site (admit, increment_weight, evict_shard_at, remove, insert_tail). Rewrite Lru::shard_weight() to read from the shadow with a Relaxed load instead of acquiring the shard's RwLock to read the LruUnit::used_weight field. This mirrors the pattern already established for shard_lens/shard_len(). Like shard_len(), the same 'best-effort, no cross-thread ordering' caveat applies: the sum of shard_weight(i) is not guaranteed to equal weight() at any single instant, since the global and per-shard atomics are updated independently with Relaxed ordering. Motivation: callers (eviction-balance heuristics, observability) that want to scan all N shards every few seconds should not pay N brief read-lock acquisitions when an atomic shadow is cheap enough to maintain on the write path. Adds a single-threaded regression test verifying the shadow stays consistent with the truth across all five weight-mutating paths.