checkpoint: unify ephemeral writes behind Write(EphemeralWriteRequest)

Mirror the persistent Write union on the shadow-branch store: a single ephemeralStore.Write(ctx, EphemeralWriteRequest) dispatches a sealed request union, replacing the two exported write methods. WriteTemporary -> writeCheckpoint (Write WriteCheckpoint{...}) WriteTemporaryTask -> writeTask (Write WriteTask{...})

Write returns WriteEphemeralResult{CommitHash, Skipped} for both requests. The EphemeralStore interface now exposes Read/Write/List, symmetric with the persistent surface. Callers (strategy, benchmarks, tests) migrated. No behavior change.