ADR 0007 — Host ownership

• 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 Vulkan/Skia host — Viewer, VulkanStartup, VulkanResources, and the runtime diagnostics — lives inside the monolith src/Lib (PackageId = FS.Skia.UI). The split package FS.Skia.UI.SkiaViewer is supposed to be the host package, but it project-references the monolith (src/SkiaViewer/SkiaViewer.fsproj → ..\Lib\Lib.fsproj) and bridges types through SceneConversion.fs. So today the host still physically lives in the monolith, and any consumer of SkiaViewer (or Elmish → SkiaViewer) transitively resolves FS.Skia.UI (see docs/reports/_baselines/2026-06-02-v3-before.md §5).

Decision

*FS.Skia.UI.SkiaViewer owns the Vulkan/Skia host.* The host modules (Viewer, VulkanStartup, VulkanResources, runtime diagnostics) move out of src/Lib into src/SkiaViewer, and the Lib.fsproj project reference is deleted. SkiaViewer depends only on the split packages it needs (Scene, KeyboardInput) — never on the monolith.

Alternatives considered

• Leave the host in the monolith and ship SkiaViewer as a thin façade (rejected). Keeps the monolith on every host consumer's transitive graph; defeats the retirement.
• Create a new FS.Skia.UI.Host package distinct from SkiaViewer (rejected). Adds a package without need; SkiaViewer is already the named host package and its consumers already reference it.

Rationale

The host is the single largest block of monolith code that has a natural split-package home. Moving it is the keystone that lets the monolith be deleted. Owning it in SkiaViewer makes the host-package's name honest and removes the SkiaViewer → Lib leak.

Affected stages

• Stage 1 (keystone): host extraction into SkiaViewer; Lib.fsproj reference removed.
• Gated by the parity oracle (ADR 0011) so the move is provably behaviour-preserving.