Which git ref namespace a KernelId::Git’s git_ref names,
chosen explicitly by the operator via the #tag= / #branch= /
#sha= fragment — never DWIM-inferred.
Human-readable enumeration of every form KernelId::parse accepts.
The macro-time rejection in declare_scheduler!(kernels = […])
cites this const verbatim. The runtime cache-lookup bails in
ktstr / cargo-ktstr cite the KTSTR_KERNEL_HINT const, a manual
mirror of this same wording — const composition cannot concat! a
const &str, only literals — so keep the two in sync when either
changes.