@spec android_resource_name(String.t()) :: String.t()

The Android res/font/ resource name for a font file: lowercase, the extension dropped, characters outside [a-z0-9_] replaced with _ (Android resource-name rules). "Georgia.ttf" → "georgia", "Inter-Regular.otf" → "inter_regular". A leading non-letter is prefixed with f_ so the name is a valid resource identifier.

This is the single implementation used both at build time (mob_dev's asset planner, when it copies a font file into res/font/) and to compute the Android half of a Mob.Theme font token (Mob.Theme.font/2) — there is deliberately no second copy anywhere that has to be kept in sync by convention. Living here (in mob core, which ships on-device) rather than in mob_dev (dev-only tooling) is what lets Mob.Theme.font/2 — app code, compiled into the release — call it directly.