SCE-002: add experiment variation-selection policies and allocation bindings

[Brad-Edwards]

Requirements

• SCE-002 — Scenario Composition, Parameterization, and Randomization

Outcome

Extend the pre-run experiment-authoring surface selected by #652 so a study declares how scenario-family variation points are fixed, enumerated, balanced, sampled, or assigned to controlled factors.

Scope

• Binding experiment factors and conditions to named scenario variation points.
• Selection policies selected by the design, including fixed, exhaustive, Cartesian, balanced/stratified, weighted, bounded random, without-replacement, and t-way coverage where supported.
• Explicit distinction among controlled factors, nuisance variables, ordinary configuration, and runtime stochasticity.
• Bounded distribution parameters and validation.
• Root stochastic-control and random-stream suite references.
• Cross-artifact validation against the referenced expanded scenario family.
• Authoring examples, schema/contract generation, fixtures, and MCP validation/scaffolding.

Acceptance criteria

• The SDL declares what may vary; the experiment declares how and why it varies.
• Every policy resolves to a declared compatible variation point and bounded domain.
• Allocation counts, factor assignments, and sampling policies are mutually consistent.
• Unsupported or unbounded policies fail admission.
• Non-random designs require no synthetic seed.
• Random designs identify the governed stochastic-control suite from Controlled Randomness And Seed Preservation (EXP-718) #274.
• Positive and negative fixtures cover every admitted policy kind.

Non-goals

• Producing concrete trial entries.
• Scheduling or executing trials.
• Hiding selection policy inside SDL defaults.

[Brad-Edwards]

🛠️ Picked up by /implement - driver codex, branch 787-experiment-variation-policies, 2026-07-28T02:52:21.928Z.

[Brad-Edwards]

gc workflow phase recorded: preflight (issue #787). Posted by the MCP server to enforce ordering between workflow steps (issue #794 MVP-2). Do not edit or delete — used by downstream tools to gate phase prerequisites.