plugin runtime

internal effect service for t3 product plugins.

it uses a deterministic, stack-safe reconciliation planner and one effect child scope per active plugin. PluginRuntime is provided through layer(), so the composing effect scope owns shutdown. updates stage changed plugins and their dependents, publish contributions atomically, and roll back without replacing the live composition when activation fails.

cordis is not a dependency. a pure-only executor was rejected because plugin lifetimes and async cleanup should be owned by effect scopes.