mount_app_mergecontract check — report sub-app entries dropped by parent-wins merge.
WhenApp.mount_apphoists a sub-app, template globals/filters/providers/etc.
already defined on the parent win over the sub-app's registration. The
dropped sub-app entries are recorded asMountAppSkipvalues on the
parent's mutable state; this check surfaces them as INFO contract issues so
the user sees what was silently shadowed.
Promote to WARNING/ERROR with
app.override_contract_severity("mount_app_merge", Severity.WARNING).