os/overlay identity units have no direct test at any tier: pithead-ssh-host-keys and pithead-mount-generator

[VijitSingh97]

Two load-bearing overlay units are only shellchecked — no tier asserts their behavior:

• `os/overlay/pithead-ssh-host-keys` (SSH host keys are baked into the image at build time — extractable from any published release, shared across machines, rotated by cache luck #894): zero references in any test file. A regression that silently regenerates host keys across A/B updates would ship undetected — and host-key churn is exactly the class of bug (SSH host keys are baked into the image at build time — extractable from any published release, shared across machines, rotated by cache luck #894/Every machine flashed from one release boots with the same machine-id #966) the battery waves kept finding by accident.
• `os/overlay/pithead-mount-generator` (Consolidate the three boot-disk/partition derivations on the lsblk approach #926 rationale: mount /data + ESP from the booted disk, never by label): only implicitly proven when a battery boot happens to succeed. Same for `os/rauc/loop-wait.sh`, and `os/build-image.sh` is tested only for `--fresh-index` flag parsing (Appliance image build 404s on a stale cached apt index when the mirror rotates a package #929).

Wanted: tier-1 shell sections where the logic permits (both scripts are argument-in/file-out enough to unit-test), plus an explicit battery assertion that host keys and machine-id survive an A/B update (the identity-survival leg added in the #966–#969 wave covers machine-id; extend it to host keys). Found by the 2026-08-14 repo audit.