Add optional saved execution data to ExecKclProject

[adamchalmers]

ExecKclProject currently cannot return the saved sketch/source data needed by native clients, or retain original filenames when uploading imports outside the entrypoint directory.

Add opt-in include_execution_data, optional response execution_data, and optional per-file original_path. Original paths are names for an uploaded in-memory filesystem; they do not authorize reads from the server filesystem. Existing requests deserialize with the new fields omitted, and response metadata is omitted unless supplied.

 ExecKclProject
+  request saved sketch/source data
+  preserve original virtual filenames

This is the protocol prerequisite for keeping Python diagnostics and sketch rendering available after the engine connection closes. The engine implementation follows separately. The command crate is bumped to 0.2.251 for that consumer update.