Phase 1: tx-size distribution (size buckets)

[bdchatham]

Part of #33. Depends on the Distribution primitive.

Problem

Calldata size is fixed per scenario (0 / ~68B / ~2KB Disperse). No way to vary payload size or sweep size buckets.

Approach

• A sizeDist (histogram/buckets) drives calldata pad length, independent of the key distribution — so a workload can sweep size while holding contention fixed, and vice versa.
• Wire into StorageRW (e.g. writeN(key, pad)) or a padded-transfer scenario.

Acceptance criteria

• Per-tx payload size drawn from configurable weighted buckets.
• Size and key distributions are orthogonal (settable independently).
• Stresses surfaced (mempool/gossip bandwidth, state-write volume) distinct from execution contention.

Design: https://github.com/sei-protocol/platform/blob/main/docs/designs/sei-load-workload-modeler.md