A marker Mutare.Phoenix.Swoosh injects for a mailer whose use Phoenix.Swoosh line
configures a layout (layout: {LayoutView, :email}).
It is not a behaviour anyone implements. Mutare's use expansion records the behaviours a
use injects and hands them to mutators as context.behaviours; this package's extension
puts this module in that set so Mutare.Phoenix.Swoosh.Layout can tell a mailer that renders
inside a layout from one that renders bare. Nothing else reads it: the marker never reaches
the metamutant source, the compiled program, or the report.
Two consequences of riding that channel, both inherited from how Mutare scopes a module's
behaviour set: the marker does not reach a nested module's calls (behaviours never inherit
inwards), and a run with --no-expand-uses drops the injected half altogether. In both cases
off falls back to its other trigger — an author-written truthy layout: assign — and mints
nothing where it cannot tell.
It exists because the use line itself is unreachable — use options are compile-time
configuration, pruned whole before any mutation could be delivered (see the "What's
deliberately out of scope" section of the README). The marker is how a compile-time fact
crosses into the runtime call the family can actually mutate: without a layout in effect,
suppressing one is an equivalent mutant, and this package does not mint those.