RetryErrorState class

Data-driven "this failed, here's why, try again" panel — composed from EmptyStatePlaceholder (same icon-badge/title/message chrome every other empty/error state in this kit already uses) plus an optional AsyncCommonButton retry action, which is what actually gives "rapid tap doesn't run the retry twice" for free (see async_common_button.dart's own re-entry guard) — this widget adds no tap-guarding of its own.

message is the only text ever rendered — RetryErrorState.fromSdkFailure only ever reads SdkFailure.message (documented there as the safe, consumer-facing string), never .cause/.stackTrace. code is a separate, optional, tappable diagnostic string (tap copies it to the clipboard) for a support conversation — fromSdkFailure derives it deterministically from kind+message via fnv1aHash, so the same error always shows the same code.

onRetry left null hides the retry button entirely rather than showing a button that does nothing.

compact controls layout only: false (default) centers the panel to fill whatever space the caller gives it (a whole screen replaced by an error state); true renders just the column, unwrapped, for embedding inline (e.g. inside a smaller panel/list empty state) — no change to EmptyStatePlaceholder itself was needed for either variant.