Function interp_nanos 

pub fn interp_nanos<'a>(
    stmts: &[Stmt<'a>],
    interner: &'a Interner,
    iters: usize,
) -> Option<u128>

RUN-ONLY tree-walking-interpreter timing — the interpreter analog of crate::vm::run_nanos. Executes the SAME statements the register-bytecode VM and the FPGA soft-core run, returning the MIN wall-time in nanoseconds over iters runs (a fresh Interpreter each run, symmetric with run_nanos building a fresh Vm). Interpreter::new is light (interned-symbol lookups only, no stdlib-prelude load), so this is a clean run-only number — the slowest tier of the cross-tier speed table (benchmarks/silicon/benchmark_timing.json): tree-walk (this) → bytecode VM (run_nanos) → native silicon (the on-chip cycle counter).