M4 epic: Observed state, difference model, and planner

[Andreas-Froyland]

Tracking issue. Dependency and status checklists only — implementation stays in narrow linked issues.

Outcome

Fleet compares desired resources with observed state and explains drift in a stable, reviewable difference model.

Supersedes

Replaces the drift-reporting portion of #2.

Depends on

• M4 desired resource schema and composition
• M2 inventory and observation model

Scope checklist

• Difference model: missing, extra, changed, unknown, unsupported
• Planner producing dependency-ordered actions from a difference set
• Dry run output usable from API, web, and fleetctl --output json
• unknown and unsupported are reported honestly rather than coerced into changed

Constrained by

• docs/PLAN.md — M4; docs/architecture/desired-state.md
• ADR-0004: desired and observed state stay separate

Non-goals

• Executing the plan (apply engine epic)
• Auto-resolving semantic conflicts