Wire SourceOS State Integrity service into boot and service lifecycle

[mdheller]

Purpose

Prepare SourceOS boot/service lifecycle for the SourceOS State Integrity daemon.

Canonical spec: SourceOS-Linux/sourceos-spec/docs/architecture/sourceos-state-integrity-layer.md
Program tracker: SourceOS-Linux/sourceos-spec#83

Service targets

• sourceos-syncd.service
• sourceos-syncd.socket
• sourceos-syncd.path
• sourceos-sync-repair.service

Requirements

• Linux-first systemd integration.
• Support user-service and system-service deployment models where appropriate.
• Support state migration hooks during package upgrades.
• Preserve durable state during daemon reinstall/upgrade.
• Expose health state for sourceos sync status.
• Emit structured journald events.
• Avoid destructive reset behavior; repair must be explicit and auditable.

Acceptance criteria

• Boot repo contains a service lifecycle design or initial service units.
• Service startup/shutdown paths do not mutate durable state without explicit repair path.
• Upgrade/migration design distinguishes durable, rebuildable, and disposable state.