Graph: IR foundation with Applicative parallelism

Evolve the graph system toward a proper IR (intermediate representation).

Phase 1 - Applicative Detection:

• Add Applicative instance that detects parallel structure
• Auto-parallelize expressions like: (,) <$> fork x <*> fork y

Phase 2 - Compile Phase:

• Separate "build graph" from "execute graph"
• Enable optimization passes between phases

Phase 3 - Optimization:

• Fuse adjacent inferences
• Eliminate dead branches
• Schedule for parallelism

See: Omni/Agent/Graph/IR.md (theoretical foundation)