PLT-465: wire key/size/op distributions into StorageRW

[bdchatham]

Implements PLT-465 (M3.3) — turns StorageRW into the two customer-named axes (key-contention + tx-size).

What (generator/scenarios/StorageRW.go)

• slot index = keyDist.SampleIndex(recordcount); op (read/write/rmw) by configured proportions; calldata pad length = sizeDist bucket draw.
• Independent seeded sub-streams per axis: dist:%d:key, dist:%d:size (existing), dist:%d:op (new, append-only to the FROZEN set) — changing one axis never perturbs another's sequence (guarded by a determinism test).
• Nil-guarded like the gas-picker: no distribution config → fixed slot 0 / rmw / empty pad, byte-identical to the PLT-461 scaffold (consumes zero randomness; account cadence untouched).
• Pad gas added as intrinsic EIP-2028 cost (4/byte) on top of the 50k base — empty pad = exactly today's 50k; large pad can't underprovision.

One-way door (flagged)

The new dist:%d:op sub-stream id is an append-only addition to the FROZEN seed→stream contract (doesn't reseed existing streams; expands the saved-replay surface).

Verify

make lint 0 issues · go build · go test -race ./... green. Tests: contention sweep, size-bucket histogram, key/size + key/op independence, op-mix, byte-identical default, additive config.

🤖 Generated with Claude Code

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📐 Design: decision brief — PLT-465 · parent design

[cursor]

PR Summary

Medium Risk
Changes load-test transaction shape, gas limits, and reproducibility semantics for StorageRW profiles; misconfiguration is mitigated by new validation, but benchmark comparisons require matching configs not just seeds.

Overview
StorageRW no longer emits a fixed slot-0 empty-pad rmw for every tx. Each transaction now optionally draws a storage slot from keyDistribution over recordCount, a calldata pad length from sizeDistribution over sizeBuckets, and read / write / rmw from operations, then calls the matching contract method. Draw order is fixed (slot → pad → operation) on the run’s single PRNG; docs and tests treat seed + full config as the replay contract (enabling an axis shifts other axes’ sequences).

Config adds frozen JSON fields recordCount, sizeBuckets, and operations (OperationMix), plus Scenario.Validate / LoadConfig.ValidateScenarios to reject half-paired axes, bad pad/keyspace sizes, and empty operation mixes. loadConfig and profile CI tests call validation at load time.

Gas stays at a 50k execution base and adds pad cost at the EIP-7623 floor (10 gas per ABI-padded byte), with validation capping pad buckets at 128 KiB to avoid admitted-but-failing txs on Sei.

Extensive unit and scenario tests cover contention, size buckets, op mix, default-path backward compatibility (no RNG use), draw-order golden sequence, and gas vs floor across pad sizes.

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