Mac-on-Linux: implement profile and host-realization lane for sourceos-shell

[mdheller]

Parent tracker: #77

Summary

Implement the profile and host-realization lane required for the Mac-on-Linux rollout of sourceos-shell across Linux dev, candidate, and stable surfaces.

Goal

Turn the current Linux scaffolds into host/profile realization work that can actually be promoted through the source-os role/channel model.

Existing upstream anchors

Relevant already-open work includes:

• #14 — Linux realization surfaces for shell services and desktop integration
• #26 — module/profile/desktop/service bootstrap
• #70 / #71 / #72 / #73 / #76 — shell module checks, docd scaffold, search-provider config, service target, and service-graph check

Workstreams

1. Profile wiring

• linux-dev, candidate, stable inclusion strategy
• host role expectations for shell-enabled machines
• service graph enablement policy per profile

2. Runtime realization

• replacement of placeholder runtime paths with real package paths
• package and service ownership once SourceOS-Linux/sourceos-shell exists
• profile-specific overrides where necessary

3. Validation

• promote current scaffold checks into host/profile/service checks
• define canary acceptance path for shell-enabled hosts
• document promotion expectations and rollback surfaces

Acceptance criteria

• profile/host realization goals are explicitly documented
• profile-wiring, runtime-realization, and validation lanes are separated and trackable
• follow-on PRs can reference this issue for host/profile work

Proposed owner

@mdheller