056 stock vs custom field: Flow PT REST finding

Q Does /schema/<Type>/fields say which fields a client can depend on finding on someone else's site?

Endpoint GET /schema/<Type>/fields ; GET /schema/<Type>/fields/<field>

Docs claim Silent. The schema is documented as a field listing with no flag distinguishing a stock field from one the site added.

Actual

each field carries: custom_metadata data_type description editable entity_type mandatory name
                    properties ui_value_displayable unique visible
each is {"value": …, "editable": …}; `visible.editable` says whether the site may hide the field

type          fields  visible.editable true  named sg_*  true and not sg_*
PublishedFile     33                      7           8  []
Version           71                     14          30  ['platform_status',
                                                          'version_sg_ai_generated_from_versions']
Shot             100                     32          39  ['platform_status']
Task              56                      8          10  []
Asset             72                     26          28  []
Project           42                      9           9  ['code']
HumanUser         65                      1           2  []
                 439                     97         126  4

sg_* with visible.editable false, on Version alone: sg_first_frame sg_last_frame sg_path_to_movie
  sg_path_to_frames sg_path_to_geometry sg_status_list sg_task sg_uploaded_movie
  sg_uploaded_movie_mp4 sg_uploaded_movie_webm sg_uploaded_movie_image
  sg_uploaded_movie_frame_rate sg_uploaded_movie_transcoding_status sg_version_type
  sg_movie_has_slate sg_movie_aspect_ratio sg_frames_have_slate sg_frames_aspect_ratio

Version.sg_status_list    visible {"value": true, "editable": false}   data_type status_list
Version.sg_uploaded_movie visible {"value": true, "editable": false}   data_type url
Project.code              visible {"value": true, "editable": true}    data_type text, name "Code"
Version.platform_status   visible {"value": true, "editable": true}    data_type text

Teaches

visible.editable is the flag, and it reads one way round. false means the site cannot hide the field, and no custom field on the probed site read false. true is "the site may hide this", which every custom field is and a few stock fields are as well, so it is a strong hint and not a proof.
The sg_ prefix decides nothing in either direction. On the probed site 126 of 439 fields are named with it and 33 of those are stock, sg_status_list and the whole sg_uploaded_movie family among them. Probe 019 explains the other half: a field created over REST is named sg_<display name> whatever the caller passes, so the prefix marks how a field was named, not who added it.
On the probed site 4 of 97 visible.editable fields are not prefixed. version_sg_ai_generated_from_versions is the reverse side of a custom multi_entity field, so it is custom under a generated name. Project.code and platform_status on Version and Shot are stock. Reading visible.editable as "custom" would have called all four wrong.
Nothing in the schema names the origin of a field. custom_metadata is "" on all 439, and visible.value is true on all 439, so neither separates anything. A client that needs certainty reads /schema/<Type>/fields on the site it is about to write to and matches on the programmatic name (probe 002), rather than deciding from a name it learned somewhere else.