Hoist the three copies of apply_state_updates (llm, agent, rag) into
graph::state_updates::apply(state, output, has_schema, updates). The
helper computes every state_updates value first, restores the transient
`output` binding, and only then writes the computed updates, so a
`state_updates: { output: "{{output}}" }` entry is no longer clobbered
by the restore step.
The per-file wrappers stay as one-line calls so their existing tests are
unchanged; new direct helper tests live in state_updates.rs, including
the regression test for the clobber.
Design: plans/map-subgraph-branches-design.md §2.6, §3.7 (C5, D3).
