PLT-465: wire key/size/op distributions into StorageRW (rebased on main)

[bdchatham]

Supersedes #54, which branched before the generator rewrite (890ce1e, #56) and cannot be rebased mechanically. Same feature, adapted to main's RNG model. #54 to be closed on merge.

What changed vs #54

The rewrite replaced named per-axis PRNG sub-streams (utils/rng, SetStream) with a single *mrand.Rand threaded per call — SampleIndex(rng, n), GenerateGas(rng). utils/rng is gone from main.

This branch adapts to that model rather than restoring the deleted package:

• OperationMix.Select(rng) takes the RNG as a parameter, matching Distribution/GasPicker.
• pickSlot/pickPad/pickOp take the RNG that CreateContractTransaction already receives.
• The two axis-independence tests are dropped. They guarded a property main no longer provides; leaving them would assert something untrue. Tracked as PLT-1023 under a later milestone.

What it does (generator/scenarios/StorageRW.go)

Each tx draws slot from KeyDistribution over RecordCount, pad length from SizeDistribution over SizeBuckets, and method from Operations. Draw order is fixed — slot, pad, operation — and documented, since one RNG is shared.

Pad gas is added as intrinsic EIP-2028 cost (4/byte) on top of the 50k base, so an empty pad is exactly the previous 50k and a large pad cannot underprovision.

Additive guarantee

Every axis is optional; every default reproduces prior behavior (fixed slot 0, empty pad, rmw). An unconfigured scenario draws no randomness — asserted by TestStorageRWDefaultPathUnchanged, which checks the RNG is left in lockstep with an untouched one at the same seed. So adding these fields cannot perturb an existing profile's workload.

Validation

Scenario.Validate() / LoadConfig.ValidateScenarios() reject a negative pad length (makeslice panic) or one over a 1 MiB cap (OOM) at load, wired into config load beside ValidateFunding.

Tests

Contention sweep at both ends, size-bucket histogram coverage with gas scaling, op-mix exclusivity and balance, weight-proportion convergence over 100k draws, seeded determinism, zero-randomness default, and omitempty round-trip. Assertions decode calldata through the binding's own ABI rather than byte offsets.

Verify

gofmt clean · go vet clean · golangci-lint run 0 issues · go test -race ./... all packages ok

[cursor]

PR Summary

Medium Risk
Changes the hot-path tx generator and load-config schema; defaults are preserved and validated, but misconfigured profiles now fail at load and configured ones change workload shape and gas.

Overview
StorageRW no longer emits a fixed slot-0 empty-pad rmw for every tx. Each transaction now draws a storage slot from KeyDistribution over RecordCount, a calldata pad length from SizeDistribution over SizeBuckets, and read / write / rmw from Operations, using the shared per-call *mrand.Rand in fixed order (slot → pad → operation). Gas is 50k base plus 4 gas per pad byte so large pads cannot underprovision.

Scenario config gains recordCount, sizeBuckets, and operations (with new OperationMix.Select), plus Scenario.Validate / LoadConfig.ValidateScenarios at load time to reject negative or >1 MiB pad lengths. main loadConfig calls validation beside existing funding checks.

Unconfigured scenarios keep the old behavior and consume no RNG on the default path; docs and tests cover contention, size histogram, op mix, determinism, and JSON omitempty.

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[bdchatham]

Closing — driving this through #54 instead. The rebased content lands there via a merge from main.