De-entangle the draft spine: lift the source-of-truth engine (the imperative
composer-runtime subscription, edit primitives, focus, edge selectors, and
per-session load/clear/stash/restore) out of ChatBar into
composer/hooks/use-composer-draft.ts.
The draft↔queue cycle is broken by making `queueEditRef` a coordinator-owned
ref ChatBar threads into the hook (explicit dep, not an implicit shared global).
The contentEditable *event* handlers stay in ChatBar (they bridge into the
trigger engine) and drive the primitives the hook exposes.
Behaviour-preserving (verbatim move); typing perf preserved. Verified: typecheck
clean, composer DOM repro tests (enter-submit, IME, slash-nav) + text-guard pass.
9ee7333e5b