refactor(formulus): subObservationMode naming follow-up

[Mishael-2584]

Summary

Follow-up to PR #591 / review: rename embedded child-form flag from
eturnOnly\ to \subObservationMode\ on \FormInitData\ and \openFormplayer\ options.

Changes

• Contract: [\FormulusInterfaceDefinition.ts](formulus/src/webview/FormulusInterfaceDefinition.ts) - \subObservationMode\ replaces
  eturnOnly.
• Native: [\App.tsx](formulus/App.tsx), [\FormplayerModal.tsx](formulus/src/components/FormplayerModal.tsx), [\FormulusMessageHandlers.ts](formulus/src/webview/FormulusMessageHandlers.ts) - wire-through + legacy
  eturnOnly\ acceptance on bridge payloads.
• Formplayer: [\App.tsx](formulus-formplayer/src/App.tsx) - \isSubObservationSession\ helper (legacy
  eturnOnly\ on init), merged draft-session nulling per review; synced interface types.

Notes

• 
  pm run generate\ in formulus failed locally (Node/ts-node ESM); injection still forwards opaque \options\ - native handler accepts both keys.
• \ormplayer_dist\ is gitignored; run
  pm run build:rn\ in formulus-formplayer before release builds.

Closes follow-up from review thread on #591.