feat: case_detail user-layer customization (user_delta_v2 superset)

Extend the user_delta_v1 envelope (ADR-024) or define user_delta_v2 to cover case_detail composition customization (currently dashboard-only).

Context: ADR-024 explicitly excludes case_detail from user_delta_v1 — case_detail user-layer rows continue using RFC 6902 ops. Workers cannot reorder/hide case_detail sections via the customize UI.

Scope:

• Decide between extending v1 envelope vs new v2.
• Loader shape-detect per (layer, surface) tuple (mirrors ADR-024 §loader dispatch).
• Validation: every referenced slug ∈ post-role baseline.
• Customize UI parity with dashboards.

Tracking origin: Stage 5 MR4 (#497 (closed)) descope. Refs &51.