fix(#114): wait for async touch/eviction goroutines before closing badger DB

[Reiers]

Closes #114.

What

state/cache.Store.Get bumps LRU asynchronously via go s.touch(c); Store.Put fires go s.evictToTarget() when live bytes exceed the soft cap. Neither goroutine was tracked, so Store.Close could return while one of them was mid-transaction against the underlying Badger DB. Once db.Close() finished, the async goroutine panicked with a nil-deref inside Badger's memTable traversal (getMemTables → memTable.IncrRef, badger db.go:731).

This has been latent on main since the persistent block cache landed. go test -count=1 ./state/cache/ reliably reproduces the panic without -race. With -race the goroutine scheduling makes it flakier but the underlying race still exists.

Fix

Track background goroutines with sync.WaitGroup. New spawnBG() gates the bg.Add(1) under a lifecycle mutex so a spawn racing Close is either counted (Close waits for it) or rejected (spawnBG returns nil). Close flips closed under the same mutex, then bg.Wait()s outside the lock, then closes the DB. Both async paths (touch and evictToTarget) use spawnBG uniformly.

Costs: one short mutex acquisition per Get (already atomic-checked before) and per over-cap Put. Happy-path behavior is unchanged.

Regression test

TestCloseWaitsForAsyncTouchAndEviction runs 32 iterations of a Get+Put+Close hammer: seed a CID, fan out 24 concurrent Gets + 24 concurrent Puts (each triggering the async LRU/eviction spawn paths at tiny soft cap), then Close. Reproduces the pre-fix panic ~5/5 without -race; passes 10/10 with the fix.

Evidence

• CGO_ENABLED=0 go build ./... — clean
• CGO_ENABLED=0 go vet ./... — clean
• go test -count=1 -timeout 300s ./... — all 55 packages ok, including state/cache in 3.7s
• go test -race -count=1 ./state/cache/ — 3/3 ok
• Old flake (go test -count=1 ./state/cache/ on pre-fix main): panicked 3/3 in local repro before the fix; 10/10 clean after.

Non-scope

• The unused evicting atomic.Bool field in Store is still dead code; leaving as-is (unrelated cleanup).
• No behavior change on the Get fast path, Put fast path, or eviction pass itself.