`download file` is parsed and built but never written: activity lands in the .mpr with no action (CE0008)

[dimenus]

Environment: mxcli v0.16.0-410-g4fda072f (built from 4fda072f), Mendix 11.13.0 project, macOS.

Steps to reproduce:

create or modify persistent entity ZZD.Doc extends System.FileDocument (
);
/
create or modify microflow ZZD.ACT_Download (
  $Doc: ZZD.Doc
)
returns Boolean
begin
  download file $Doc;
  return true;
end;
/

Run mxcli exec script.mdl -p project.mpr, then describe microflow ZZD.ACT_Download and mx check project.mpr.

Expected behavior: The activity is stored as a Microflows$DownloadFileAction and round-trips as download file $Doc;.

Actual behavior: mxcli check passes and mxcli exec reports Created microflow with no warning. The activity is written with no action at all:

begin
  ...
  -- Empty action
  return true;
end;

mx check then reports:

[error] [CE0008] "No action defined." at Action activity 'Activity'

The statement is accepted at every stage that reports anything, and disappears at the one stage that does not. download file $Doc show in browser; behaves the same.

Root cause: the write path has no case for it. actionToGen in mdl/backend/modelsdk/microflow_write.go switches on *microflows.<Action> and covers ~80 action types; DownloadFileAction is not among them (grep -c DownloadFileAction microflow_write.go → 0), so the ActionActivity is emitted with a nil Action.

Everything either side of the writer is already in place, which is what makes this easy to miss:

• grammar/visitor: downloadFileStatement → ast.DownloadFileStmt (mdl/visitor/visitor_microflow_actions.go:1193)
• flow builder: addDownloadFileAction builds a correct microflows.DownloadFileAction (mdl/executor/cmd_microflows_builder_calls.go:845)
• read path: actionFromGen handles it (mdl/backend/modelsdk/microflow_read_actions.go:376), covered by TestActionFromGen_DownloadFile
• formatter: mdl/executor/cmd_microflows_format_action.go:737

So there is a builder test and a reader test, and the only untested link is the one that is missing. docs/11-proposals/PROPOSAL_microflow_download_file_statement.md:53 lists "Builder/writer coverage for DownloadFileAction" as required — the builder half landed, the writer half did not.

Suggested fix: add a case *microflows.DownloadFileAction to actionToGen in microflow_write.go, writing FileDocumentVariableName, ShowFileInBrowser and ErrorHandlingType. Note the storage key is ShowFileInBrowser, not ShowInBrowser — the reader test above documents that legacy reads the wrong key.

A round-trip test (exec → describe) would catch this class generally; a reader-only test cannot, because it starts from BSON that the writer never had to produce.