Model server startup as a typed boot dataflow graph

Server startup currently relies on statement order and shared mutable state to communicate lifecycle dependencies.

Introduce a small generic pkg/boot package and move server lifecycle ownership into focused components under components/server. Typed outputs should carry ready values and declare graph edges at the same time.

Done when:

• The graph validates missing dependencies and cycles before startup.
• Independent components start in parallel.
• A consumer runs only after all of its producers publish successfully.
• The same edges produce reverse dependency-order shutdown.
• commands.Server remains the CLI policy, configuration, signal, and runtime-handoff layer.
• Build saga recovery waits for BuildKit, the OCI registry, and registry host mapping.
• Build and deployment handlers are exposed only after every boot capability their request paths need is available.
• A disabled optional component, such as BuildKit, resolves successfully without holding the graph open.
• RFD-88 records the refined boot-only model and no longer promises runtime readiness conditions.

Implementation: mirendev/runtime#1096