Macro require 

macro_rules! require {
    ($spec:literal) => { ... };
}

require!(<spec>) — build a RequiredCapability from a string-literal spec. Panics at construction on malformed input (matches the substrate plan’s “validates shapes at parse time” contract for the macro family).

Forms

require!("hardware.gpu");                  // axis presence
require!("software.daemon:postgres");      // axis value
require!("hardware.gpu.vram_gb >= 24");    // numeric ≥
require!("hardware.cpu_cores <= 64");      // numeric ≤
require!("software.runtime == \"cuda-12.4\"");  // string equality

Reserved-prefix tags (causal:, scope:, etc.) are rejected — require!("scope:prod") panics with CapabilityTagError::ReservedPrefix. Use require_axis_value! if a reserved-prefix concept needs to land in an intent registry (it shouldn’t — those prefixes are substrate-private).