evaluator/ is the calculus; runtime/ is the plumbing

The CBPV machine and the OS plumbing now live in separate module trees, so “the calculus is the cheap part” is a fact of the file layout rather than a claim about it. core/src/evaluator/ had grown to ~9.2k lines, over budget relative to the theory; inspection showed the actual machine — comp, expr, val, call, case, pattern, scope, trampoline, capture, redirect frames, audit — is ~2.6k, and the other ~6.6k was command/pipeline/transport machinery already coupled to it through clean seams. This relocates that machinery to core/src/runtime/.

• evaluator/ is exactly the machine now. command/, command.rs, command_call.rs, pipeline/, pipeline.rs, and transport.rs moved to runtime/; evaluator/ holds only the eleven machine modules. Zero lines of logic changed — paths, mod declarations, and two visibility modifiers only (evaluator-machine, cbpv).
• The up-seam is three verbs. The runtime re-enters evaluation through call::invoke, eval_block, and absorb_tail — the designated re-entry points. It also reaches comp::eval_comp, audit, apply, and redirect, each genuinely used elsewhere crate-wide (builtins, the crate-root child-eval runner, hosts via re-exports), so none narrows below pub(crate) without breaking a real caller. Rust’s pub(crate) cannot make privacy enforce a three-function seam while those helpers serve other consumers; the seam is documented and as narrow as the existing coupling allows.
• The down-seam is narrow, not singular. The pipeline sub-seam is the single call comp → pipeline::run_pipeline, as intended (pipeline-execution). Other evaluator→runtime edges remain — call → command_call::run_call and command::EvalRedirect, redirect → command guards, evaluator → transport::dispatch — pre-existing and not collapsible without a logic change. runtime.rs names them rather than pretending one edge exists.
• The mutual recursion is irreducible. A stage body contains closures, so the stage runner must re-enter the evaluator; the seam exists precisely because the recursion cannot be cut. The split makes the boundary visible, not absent.

Realistic end state, achieved: evaluator/ at the machine size and runtime/ holding the fd/pgid/signal floor that any Unix shell pays.