Replaces the workflow_id-on-every-event approach (#13684, reverted in
#13901) with a generic metadata envelope captured at submission and
injected at the server-side send chokepoint.
- POST /prompt accepts an opaque ``extra_data.metadata`` dict (falls
back to synthesizing ``{"workflow_id": <id>}`` from
``extra_pnginfo.workflow.id`` so existing frontends keep working).
- ``PromptServer`` owns a ``prompt_id -> metadata`` map populated at
submission, drained when the prompt finishes. ``send_sync`` injects
the envelope into any outbound payload that carries a ``prompt_id``,
including the ``(preview_image, metadata_dict)`` tuple used by
``PREVIEW_IMAGE_WITH_METADATA``. WS reconnect path carries it too.
- Pure helpers live in ``app/prompt_metadata.py`` so the execution
layer never depends on workflow concepts and the helpers can be
unit-tested without torch.
Execution layer (``execution.py``, ``comfy_execution/*``) and the jobs
API are unchanged. Backward compatible: existing fields and shapes are
preserved, only an additional ``metadata`` field is attached when
present.
