Merge: mask a whole category, not just one instance

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

# Conflicts:
#	apps/darkroom-desktop/Cargo.toml
#	docs/traceability.md

apps/darkroom-desktop/Cargo.toml

@@ -6,7 +6,7 @@ rust-version.workspace = true
 license.workspace = true
 
 [dependencies]
-dr-ui.workspace = true
+dr-ui = { workspace = true, features = ["scene-model"] }
 # For the panic hook and the log sink, directly rather than through dr-ui:
 # both have to be installed before `dr_ui::run`, because a panic during startup
 # is exactly the one they exist to catch and record (NFR-OPS-1, NFR-OPS-2).