Core names a cookie that only one implementation reads #1275

The problem

COOKIE_MTM_PREF, the name of the cookie a preference platform writes the Model Terms for Marketing preference into, sits in core's constants. It is not in main today. It arrived with the stack's own Model Terms signal module in a prior pull request of the stack, the one that adds the module documentation set (#1047). This issue corrects that prior pull request, which cannot change at this stage, rather than anything in main, and the reason is elegance alone, that core should name no one implementation's cookie. The Model Terms permission signal module, crates/permission-signal/mtm, is the only code that reads it. The cookie is particular to that one implementation of a permission signal. It is not a standard core supports, and nothing in core should know its name.

The change

The constant moves to the signal crate with its comment, and core names one implementation's cookie less. Nothing else changes. The cookie's name is the same, the module reads it as before, and the Axum adapter's test of the signal takes the name from the crate. Three files, 8 lines added and 9 removed.

Where it is

Draft on the fork, jwrosewell#12, on top of auction telemetry as a module (fork pull request 11).