Platform seams

Replaces workspace-app-composition, whose subject (defineWorkspace, the workspace singleton, environment factories, the daemon) was deleted with the workspace plane. The seam below is the part that survived.

Storage is not a seam

Every build opens its own store. A host serves bundles and brokers credentials and owns no application data (ADR-0226), so there is no build where data lives somewhere else, and a #platform/* seam for storage is the thing to delete rather than to route. Honeycrisp calls openBrowserStore in every build including the Tauri one, and apps/honeycrisp/src/lib/application-platform.ts states that as a refusal with its reasons.

What is left behind a seam is how a build gets a bearer and which deployment it talks to, plus native capability. Honeycrisp declares exactly two: #platform/auth and #platform/instance.