Spec design for fx parity implementation

[aryeko]

Story

• fx Parity-Capable App #72

Motivation

fx variant must preserve contract while using its DI style.

Scope

In:

• Scenario mapping
• Shared-contract usage plan
• Verification order

Out:

• Benchmark policy changes

Requirements

• Define parity-preserving handler design
• Define seed/state behavior

Acceptance criteria

• fx design covers parity and run-single requirements

Verification

Commands:

• make -n parity-check

CI checks:

• (none)

Evidence:

• fx design mapping

Artifacts

• Design consumed by T-031..T-033

Spec reference

• .sisyphus/plans/apps-implementation-roadmap.md
• Section: ST-009
• Quote: fx Parity-Capable App

Dependencies

Blocked by:

• Document anti-drift usage guidance #78