World identity derives from the whole scenario, not the variant

Amends ADR 0001. A world's identity is now h(canonical_hash(scenario), replicate_seed, population_hash) rather than h(variant_id, replicate_seed, population_hash). Price, audience weights and interventions all live on the scenario, not the variant, so keying identity on the variant collapsed distinct cells: a price sweep over one concept at two prices produced two worlds for four cells, and resume would have treated the second price point as already computed.

The world seed keeps ADR 0001's derivation, h(replicate_seed, variant_id). Worlds of one variant therefore share their random draws across prices and conditions, so comparing two price points isolates the price effect instead of mixing it with sampling noise.

Consequences

A run or grid refuses repeated replicate seeds, repeated scenarios, one variant identifier naming two different concept cards, and scenarios with differing tick units — each of which would otherwise produce colliding or incomparable worlds. Changing the canonical form of a scenario changes every world identity; as with ADR 0001, treat the derivation as frozen once traces exist.