PLT-455: Distribution config primitive + discriminator

[bdchatham]

Implements PLT-455 — the reusable keyspace-distribution config primitive for the sei-load workload modeler. Locks the JSON wire format (a one-way door); sampling math is deliberately deferred to PLT-460.

What

• config/distribution.go — a Distribution tagged type mirroring config/gas.go's GasPicker discriminator exactly: Name-tagged two-pass UnmarshalJSON, unexported indexSampler interface + compile-time assertions, loud default: error on unknown kinds. Kinds: "uniform", "zipfian" (param theta, range-validated at parse time).
• Optional KeyDistribution / SizeDistribution (*Distribution, omitempty) on config.Scenario, nil-guarded like the existing gas-picker field.

Locked wire format (one-way door)

Discriminator Name; kinds uniform/zipfian; zipfian param theta. Frozen — documented in code.

Deferred to PLT-460

SampleIndex is a clearly-marked placeholder (return 0, nil) — the YCSB precomputed-zeta zipfian + uniform draw (using the PLT-456 seeded RNG) lands there. Confirmed no existing caller can reach it.

Additive

A config with no distribution fields round-trips byte-identically (golden test against profiles/conflict.json); no existing profile changes meaning.

Tests / review

go build + go test -race ./config/... green. Idiom cross-review: clean (faithful gas.go mirror).

Decision brief: designs/sei-load-workload-modeler/PLT-455-distribution-discriminator.md (sei-protocol/bdchatham-designs).

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[cursor]

PR Summary

Low Risk
Additive config and parsing only; no sampling logic or callers yet, with backward-compatible round-trip behavior covered by tests.

Overview
Adds a keyspace distribution config primitive (patterned after GasPicker) and wires it into load scenarios as optional keyDistribution / sizeDistribution fields.

The new Distribution type uses a frozen JSON discriminator (Name: uniform or zipfian), validates zipfian theta at parse time, and rejects unknown kinds. SampleIndex is intentionally stubbed (0, nil) until PLT-460 implements actual uniform/zipfian draws.

Tests cover unmarshaling/validation and prove existing profiles without these fields still parse and remarshal without adding distribution keys (golden check on profiles/conflict.json).

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