SESSION-AUTHORITY-DOCTRINE §3 names /etc/pam.d/souveraine-stepup by path.
Root-owned config can only arrive by package: rootfs-overlay/ reaches a
device on flash and the phone does not flash.
auth_self, never auth_admin — step-up re-proves the user to themselves, it
does not escalate. The leading '-' on pam_fpc_polkit.so loads the fingerprint
factor only where it is installed, so one file serves phone and laptop.
Inert until a consumer calls it; no src/ path does yet.
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