evaluator: body_origin manual LIFO restore → RAII guard (future-proofing)

[dean0x]

Review source: .devflow/docs/reviews/fix-v040-dogfood-remediation/2026-07-20_1305

PF-012 class: Future-proofing against concurrent state mutation.

• ISS-10 body_origin manual LIFO restore at 3 exit sites → RAII guard (crates/mds-core/src/evaluator.rs:645-777) — correct today (3 reviewers verified); future-proofing only; avoids PF-012 class risk

Current behavior is verified sound; this refactor prevents accidental regressions if evaluator exit paths change.