Constant SIGNATURE_WINDOW_SECONDS 

pub const SIGNATURE_WINDOW_SECONDS: u64 = 300; // 300u64

I2 (#1971): how far, in seconds, a request’s timestamp may be from the callee’s clock, either way. A requirement, not a tuning knob (ADR per-edge-call-signature): it bounds how long a captured request can be replayed, and nothing else does.