#1640 extracted the voice subsystem to server/voice/ (a peer of session.ts);
#1644 extracted the checkout read side to session/checkout/. The two carves
landed in inconsistent homes, so there was no single place to find "what has
session.ts been carved into."
Relocate server/voice/ -> server/session/voice/ so all session subsystems share
one home alongside session/checkout/. Pure relocation: git-tracked renames with
import-depth bumped one level (../ -> ../../); the only external touch point is
the import path in session.ts. Behavior preserved verbatim — the 17 voice unit
tests pass unchanged before and after.
Also record the actual shipped layout and the next carve (the checkout mutation
handlers still inline in session.ts -> session/checkout/checkout-session.ts) in
the decomposition plan, and fix its now-stale voice path.
