Macro record_kernel_variant 

macro_rules! record_kernel_variant {
    ($variant:expr, $($arg:tt)+) => { ... };
}

Record a selected kernel implementation and lazily formatted reason on the active executor op-span.

Formatting is skipped entirely unless a span is active (kernel_variant_tracing_enabled), so instrumented dispatch sites stay allocation-free on the hot decode path when tracing is off.