This commit implements the download flow for SaveAs and ExportCsv per the
USER RULING on improvise-2mqz. The correct shape is:
- A request frame asks the host for BYTES
- ServerFrame returns the bytes (or Rejected)
- Browser triggers downloads via Blob + <a download>
Key changes:
Protocol layer (improvise-protocol):
- Add Request::DownloadDocument and Request::DownloadCsv variants
- Add ClientFrame::DownloadDocument and ClientFrame::DownloadCsv
- Add ServerFrame::DocumentBytes and ServerFrame::CsvBytes
- Add KeyOutcome::LocalThenRequest variant for operations that need
both local fold (mode change, buffer clear) AND a server request
- Update stranding() to unstrand SavePrompt and ExportPrompt modes
- Update keymap to use rebind() instead of commit() for commit-save
and commit-export (because LocalThenRequest can't be in a Sequence)
Host layer (improvise-command):
- Handle new ClientFrames in host.rs returning bytes or Rejected
- Add Rejection::ExportFailed variant for CSV export failures
- Update CommitSave and CommitExport to clear their buffers
- Make render_csv() public in improvise-io for host use
WASM client layer (improvise-wasm-client):
- Add PendingDownload type with filename, bytes, and kind
- Store pending downloads for JavaScript retrieval via take_pending_download()
Browser layer (web/app.js):
- Add triggerDownload() function using Blob + <a download>
- Poll for pending downloads after frame handling
This unstrands SavePrompt and ExportPrompt which were previously refused
by stranding() due to missing wire forms.
Refs: improvise-2mqz
Co-authored-by: Ed Langley <fiddlerwoaroof@users.noreply.github.com>
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