Operator CLI parser + verb runtime for fathomdb.
Surface owned by dev/interfaces/cli.md. Phase 10a wires the parser
scaffold to real engine seam calls: doctor check-integrity,
doctor safe-export, doctor trace, recover --rebuild-projections,
recover --rebuild-vec0, and recover --excise-source invoke the
corresponding [fathomdb::Engine] methods and serialize the typed
report under the per-verb JSON discriminator.