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-generator hot paths and the frozen RNG stream-id set (replay surface expands); default-path compatibility is explicitly tested but misconfigured large pads are now possible until validation catches them.

Overview
StorageRW moves from a fixed single-slot rmw scaffold to configurable key contention, tx size, and read/write/rmw mix. Each tx draws a slot via KeyDistribution + recordCount, pad length via SizeDistribution + sizeBuckets, and an operation via weighted operations; key, size, and op use separate RNG sub-streams (dist:%d:key|size|op) so tuning one axis does not shift the others.

Scenario JSON gains optional recordCount, sizeBuckets, and operations; OperationMix.Select handles weighted op choice with nil/zero-weight fallback to rmw. ValidateScenarios rejects bad sizeBuckets (negative or over 1 MiB) at load time. Gas stays at a 50k base plus 4 gas per zero pad byte (EIP-2028). With no distribution config, behavior stays byte-identical to the prior fixed slot 0 / empty pad / rmw path.

The new dist:%d:op stream id is an append-only addition to the frozen seed contract; generator wiring binds it alongside existing distribution streams.

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