test(store): wait for append before checking head

[loglapa]

Overview

Stabilizes TestStore by waiting for the asynchronous append pipeline before asserting the contiguous head. Store.Append intentionally returns after enqueueing writes (see #244), and GetRange can observe pending headers before advanceHead updates contiguousHead; the previous test therefore compared the new range with the old head intermittently.

The production behavior is unchanged.

Validation

• Reproduced the original test failure 3 times in 200 runs
• Updated test: 500/500 focused runs pass
• go test ./store
• go test -race ./store
• go vet ./...
• golangci-lint run (0 issues)
• git diff --check

The repository-wide suite is currently blocked on an unrelated deterministic panic introduced on main by merged #406 (TestExchangeServer_partialRangeNotExpanded calls the newly required Tail method through a nil embedded store). It reproduces on both Go 1.26/darwin and Go 1.27/linux; details are posted on #406. This PR changes only store/store_test.go.