fix(engine): send clients the composed Telex text, not the keys

For Vietnamese the preedit is the map key ('as', 'oong'); only the popup
and the commit path looked it up. Clients with inline preedit therefore
showed 'as' while Enter committed 'á'. impre() now maps once for every
reader, and snapshot() no longer maps a second time.

tests/unit_test.c

@@ -94,6 +94,7 @@ static const struct ct_test tests[] = {
 	{ "engine/rejects-stale-dictionary", engine_rejects_stale_dictionary_results },
 	{ "engine/active-owner-caret", engine_active_owner_caret },
 	{ "engine/popup-preedit-capability", engine_popup_preedit_capability },
+	{ "engine/vietnamese-client-preedit", engine_vietnamese_client_preedit },
 	{ "engine/commit-contract", engine_commit_contract },
 	{ "engine/language-switch-state", engine_language_switch_state },
 	{ "engine/telex-history-bound", engine_telex_history_bound },