chore: fix pre-existing main breakages blocking clean overlay composition (server-session diff_version type clash; i18n 'am' locale parity)

[aarontrowbridge]

Problem — Two failure classes exist on pristine origin/main (verified by multiple casts with stash-verified base runs) and they surface as composition errors in every overlay PR that touches the session surfaces:

1. packages/... server-session.ts references a diff_version field the overlay's SessionCache type does not carry (typecheck breaks in materialized trees).
2. i18n parity test fails on the 'am' (Amharic) locale — missing keys.
   Why it matters — Every overlay PR inherits red composition; reviewers cannot distinguish overlay breakage from base breakage without a stash dance (done three times today).
   Fix — Reconcile the SessionCache type with the canonical server-session shape; restore the missing 'am' locale keys.
   Source — Documented on PR Re-home the session & device lifecycle base contract onto the app overlay (canonical v1.18.29) #819's review notes; independently hit by the slice-1 fork cast and the re-home cast.