ADR 0008 — Scene-vocabulary single source

• Status: Accepted
• Date: 2026-06-02
• Decision source: the V3 modular-distribution implementation plan (docs/reports/2026-06-02-v3-modular-distribution-implementation-plan.md, §0.4 + Stage 1) and feature 048-v3-retirement-baseline (research.md D8).

Context

The scene vocabulary is defined twice: in the split src/Scene/Scene.fsi (structured Scene = { Nodes: SceneNode list }) and in the monolith src/Lib/Library.fsi (an opaque Scene plus duplicate value types). The before-baseline (docs/reports/_baselines/2026-06-02-v3-before.md §4) records 34 types defined in both files, plus duplicate Scene/Picture/SceneNode and the Colors/Paint/Path/ Scene modules. The host bridges the two representations via src/SkiaViewer/SceneConversion.fs.

Decision

*FS.Skia.UI.Scene is the canonical, single source of the scene vocabulary.* The monolith's duplicate scene types are deleted, the host is retyped directly onto the FS.Skia.UI.Scene types, and SceneConversion.fs is removed — no permanent conversion shim. A temporary build flag may gate the cut-over until parity is signed off (ADR 0011), then it is removed.

Alternatives considered

• Keep SceneConversion.fs as a permanent adapter (rejected). Institutionalizes the duplication and a per-frame conversion cost; the duplication is the very thing the retirement removes.
• Make the monolith's Scene the canonical type (rejected). The monolith is being deleted; the canonical vocabulary must live in a surviving split package, and the structured FS.Skia.UI.Scene.Scene is the richer, already-public representation.

Rationale

A single structured Scene type eliminates the 34-type duplication, the opaque/structured split, and the conversion shim in one move. It is the precondition for deleting src/Lib.

Affected stages

• Stage 1 (keystone): delete the monolith's duplicate vocabulary; retype the host onto FS.Skia.UI.Scene; remove SceneConversion.fs.
• Verified byte-identically by the parity oracle (ADR 0011, SC-003).